approved by Finchinator
What is Dream World OU?
Pokemon Dream World was a browser game released alongside Pokemon Black and White where trainers could, among other things, visit friends' houses, put their Pokemon to bed, and grow berries in their garden. But, there was one area that truly changed competitive Pokemon, the Dream Tree. The Dream Tree was where players could exchange berries and items for Pokemon that came equipped with their Hidden Abilities, a new feature added in Black and White. New areas including new batches of available Pokemon would be released throughout the lifespan of BW1 and partially into BW2. During this time, a new competitive format was created, Dream World OU, a take on BW OU where all Pokemon with their Hidden Abilities were legal to use, even before they could be obtained in-game. Unfortunately, this metagame's playerbase eventually petered out as more and more Hidden Abilities became available in standard play. However, although not as radically different from standard OU as it was in the past, this format still has something unique to offer as a handful of Pokemon never were released with their Hidden Abilities in Generation 5 and some Pokemon never had their Dream World HAs ever made obtainable. So, if you would like to try this tier out, the rest of this post will show you how you can still play this unique metagame as well as briefly go over the relevant new additions to the tier.
How to Play Dream World OU 
DW OU is only playable through direct challenges on Showdown.
This first code is just current BW OU but with all the unreleased HAs made legal.
CHALLENGE CODE:
Alternatively, if you would like to play something with a more limited banlist and more akin to the original DW OU, you can use this code:
And finally, mainly because I think Lightning Rod Zapdos is pretty neat, here is DW VGC:
if you spot anything wrong with any of these codes please lmk
Notable New Additions 
Chandelure
Shadow Tag
Pretty much the first Pokemon anyone thinks of when Dream World abilities are mentioned and it's easy to see why. Chandelure is by far the most offensively inclined trapper in BW, sporting the frankly absurd combination of 145 SpA, a nearly unresisted STAB combo, and, of course, Shadow Tag. Obviously all this makes it just a bit too much for standard play, but a truly fearsome revenge killer in formats where it is actually legal.
Serperior
Contrary
As you probably already know, a solid supporting movepool, good base speed, and, most importantly, Contrary Leaf Storm turns Serperior into an offensive powerhouse. On top of that, Black and White's mechanics turn up the intensity just that bit more with Leaf Storm still at its original 140 BP as well as Glare, though now only 90% accurate, is now able to paralyze every single Pokemon in the game, regardless of typing.
Zapdos
Lightning Rod
The fifth generation of Pokemon has always been a blotch on Zapdos' otherwise stellar OU record in part due to the difficulty it has had competing with the likes of Thundurus and Rotom. Luckily, Lightning Rod levels the playing field a bit more by giving Zapdos a handy Electric immunity plus a situational SpA boost allowing it to further leverage its superior bulk and recovery compared to its competition, giving it a better matchup against said Electrics and making Zapdos a more threatening Pokemon in general.
Feraligatr
Sheer Force
Feraligatr currently holds a small niche as a heavy hitter on rain teams with its access to both Dragon and Swords Dance allowing to stack damage boosts with STAB, rain, and set-up. Well you can now add one more to that list with Sheer Force further boosting its Waterfalls to an effective 104 BP as well as boosting its most common coverage options, hopefully bringing the Gatr's power level a bit closer to the other rain monsters in the tier.
Heatran
Flame Body
Of the Pokemon featured here, Heatran is obviously the most viable one in current BW and is also the only Pokemon here that is actually less likely to use its new ability over its original one. Nevertheless, Flame Body adds a new wrinkle to Heatran's already strong defensive profile giving it a chance to completely cripple physical attackers while still forcing opponents to respect the possibility of Flash Fire until its ability is revealed.
Suicune
Water Absorb
Suicune is already one of the bulkiest Pokemon available in Generation 5, but has always been held back the altered sleep mechanics' effect on RestTalk sets as well as the general increase in power across the board has left the Aurora Pokemon struggling to compete both with either its offensive or defensive sets. Water Absorb gives Suicune a valuable immunity as well as a situational, yet nonetheless valuable source of recovery for a Pokemon that has always had difficulty staying healthy enough to actually utilize its good traits.
Moltres
Flame Body
Another new Flame Body Pokemon and another niche rain mon. In current BW, Moltres is mainly known for launching Hurricanes and Fire Blasts from its strong 125 base SpA stat as well as its ability to threaten and check the likes of Ferrothorn, Volcarona, and Scizor. The addition of Flame Body makes Moltres that much more effective as a burn spreader, allowing the phoenix to further frighten the physical attackers of the tier and potentially punish stray U-Turns and other weak hits.
- - -
Articuno - Snow Cloak
Zapdos - Lightning Rod
Moltres - Flame Body
Meganium - Leaf Guard
Typhlosion - Flash Fire
Feraligatr - Sheer Force
Raikou - Volt Absorb
Entei - Flash Fire
Suicune - Water Absorb
Regirock - Sturdy
Regice - Ice Body
Registeel - Light Metal
Heatran - Flame Body
Serperior - Contrary
Emboar - Reckless
Samurott - Shell Armor
Chandelure - Shadow Tag
Volcarona - Swarm
- - -
What is Dream World OU?Pokemon Dream World was a browser game released alongside Pokemon Black and White where trainers could, among other things, visit friends' houses, put their Pokemon to bed, and grow berries in their garden. But, there was one area that truly changed competitive Pokemon, the Dream Tree. The Dream Tree was where players could exchange berries and items for Pokemon that came equipped with their Hidden Abilities, a new feature added in Black and White. New areas including new batches of available Pokemon would be released throughout the lifespan of BW1 and partially into BW2. During this time, a new competitive format was created, Dream World OU, a take on BW OU where all Pokemon with their Hidden Abilities were legal to use, even before they could be obtained in-game. Unfortunately, this metagame's playerbase eventually petered out as more and more Hidden Abilities became available in standard play. However, although not as radically different from standard OU as it was in the past, this format still has something unique to offer as a handful of Pokemon never were released with their Hidden Abilities in Generation 5 and some Pokemon never had their Dream World HAs ever made obtainable. So, if you would like to try this tier out, the rest of this post will show you how you can still play this unique metagame as well as briefly go over the relevant new additions to the tier.
How to Play Dream World OU DW OU is only playable through direct challenges on Showdown.
This first code is just current BW OU but with all the unreleased HAs made legal.
CHALLENGE CODE:
/challenge gen5ou@@@+Unreleased,!Obtainable MiscAlternatively, if you would like to play something with a more limited banlist and more akin to the original DW OU, you can use this code:
/challenge gen5ubers@@@+Unreleased,-Arceus,-Darkrai,-Deoxys,-Deoxys-Attack,+Deoxys-Speed,+Deoxys-Defense,-Dialga,-Giratina,-Giratina-Origin,-Groudon,-Ho-Oh,-Kyogre,-Lugia,-Mewtwo,-Palkia,-Rayquaza,-Reshiram,-Shaymin-Sky,-Zekrom, -Soul Dew, -Swift Swim ++ Drizzle,!Obtainable MiscAnd finally, mainly because I think Lightning Rod Zapdos is pretty neat, here is DW VGC:
/challenge gen5vgc2013@@@+Unreleased,!Obtainable Misc if you spot anything wrong with any of these codes please lmk
Notable New Additions Chandelure
Shadow Tag
Pretty much the first Pokemon anyone thinks of when Dream World abilities are mentioned and it's easy to see why. Chandelure is by far the most offensively inclined trapper in BW, sporting the frankly absurd combination of 145 SpA, a nearly unresisted STAB combo, and, of course, Shadow Tag. Obviously all this makes it just a bit too much for standard play, but a truly fearsome revenge killer in formats where it is actually legal.
Serperior
Contrary
As you probably already know, a solid supporting movepool, good base speed, and, most importantly, Contrary Leaf Storm turns Serperior into an offensive powerhouse. On top of that, Black and White's mechanics turn up the intensity just that bit more with Leaf Storm still at its original 140 BP as well as Glare, though now only 90% accurate, is now able to paralyze every single Pokemon in the game, regardless of typing.
Zapdos
Lightning Rod
The fifth generation of Pokemon has always been a blotch on Zapdos' otherwise stellar OU record in part due to the difficulty it has had competing with the likes of Thundurus and Rotom. Luckily, Lightning Rod levels the playing field a bit more by giving Zapdos a handy Electric immunity plus a situational SpA boost allowing it to further leverage its superior bulk and recovery compared to its competition, giving it a better matchup against said Electrics and making Zapdos a more threatening Pokemon in general.
Feraligatr
Sheer Force
Feraligatr currently holds a small niche as a heavy hitter on rain teams with its access to both Dragon and Swords Dance allowing to stack damage boosts with STAB, rain, and set-up. Well you can now add one more to that list with Sheer Force further boosting its Waterfalls to an effective 104 BP as well as boosting its most common coverage options, hopefully bringing the Gatr's power level a bit closer to the other rain monsters in the tier.
Heatran
Flame Body
Of the Pokemon featured here, Heatran is obviously the most viable one in current BW and is also the only Pokemon here that is actually less likely to use its new ability over its original one. Nevertheless, Flame Body adds a new wrinkle to Heatran's already strong defensive profile giving it a chance to completely cripple physical attackers while still forcing opponents to respect the possibility of Flash Fire until its ability is revealed.
Suicune
Water Absorb
Suicune is already one of the bulkiest Pokemon available in Generation 5, but has always been held back the altered sleep mechanics' effect on RestTalk sets as well as the general increase in power across the board has left the Aurora Pokemon struggling to compete both with either its offensive or defensive sets. Water Absorb gives Suicune a valuable immunity as well as a situational, yet nonetheless valuable source of recovery for a Pokemon that has always had difficulty staying healthy enough to actually utilize its good traits.
Moltres
Flame Body
Another new Flame Body Pokemon and another niche rain mon. In current BW, Moltres is mainly known for launching Hurricanes and Fire Blasts from its strong 125 base SpA stat as well as its ability to threaten and check the likes of Ferrothorn, Volcarona, and Scizor. The addition of Flame Body makes Moltres that much more effective as a burn spreader, allowing the phoenix to further frighten the physical attackers of the tier and potentially punish stray U-Turns and other weak hits.
- - -
Articuno - Snow Cloak Zapdos - Lightning Rod Moltres - Flame Body Meganium - Leaf Guard Typhlosion - Flash Fire Feraligatr - Sheer Force Raikou - Volt Absorb Entei - Flash Fire Suicune - Water Absorb Regirock - Sturdy Regice - Ice Body Registeel - Light Metal Heatran - Flame Body Serperior - Contrary Emboar - Reckless Samurott - Shell Armor Chandelure - Shadow Tag Volcarona - Swarm