Ho-Oh is a seriously underrated Uber. 50% burn is nothing to play with. It is an extreme staller, as well, with Recover, 106 base HP, and 154 base Special Defense. The trick to Ho-Oh is to try and burn any physical attackers you can, and then use your massive Special Defense to keep you alive on that front. Unlike Lugia, who, with a Calm Mind set, likes some Defense EVs and tries to boost up in the face of physical attackers, Ho-Oh has the ability to severely cripple them. However, its mediocre Speed (tied with Kyogre, Groudon, and Deoxys-D as the slowest Uber) is what holds it back. It has trouble with the physical attackers, because they're likely faster and carry Rock Slide, and its weaknesses to Thunder, Thunderbolt, and Surf hurts its ability to wall special hits. Ho-Oh should always be used with Groudon. Kyogre is one of the most common Pokemon in Ubers, and you don't want Drizzle blocking your Sacred Fire. With Groudon, you can overwrite Drizzle with Drought to boost your Sacred Fire instead of weaken it. This also makes enemy Surfs be at half power, and Thunder gets 50% accuracy.
Use in conjunction with Groudon for boosting Sacred Fire. Calm Mind up, burn everything in sight. Sacred Fire is what makes Ho-Oh so deadly; nothing can switch into this bird without fear of being burned. Thunder is nice to hit Kyogre, the most common switch into Ho-Oh, but Thunderbolt helps against enemy Ho-Oh, and does a bit more to Rayquaza (try and Burn it first, then you can survive even a Choice Band Rock Slide if you have a lot of HP EVs).
Ho-Oh's MASSIVE Attack, myriad physical moves, and Sacred Fire make it a decent Choice Bander. Hidden Power Flying is a safe move to use as it hits almost everything in the uber metagame with at least neutral damage except for Metagross and the rare Skarmory, both of which rarely switch in for fear of Sacred Fire. However, Shadow Ball hits more things super effective, and Earthquake has roughly the same power (5 less, with STAB) as Hidden Power Flying, so it would primarily be for when you can't decide between trying to hit Rayquaza or Kyogre, or something. Hidden Power Rock, however, allows you to OHKO enemy Ho-Oh, and possibly OHKO Rayquaza. Hidden Power Rock has more uses that aren't covered by your other moves better, but Hidden Power Flying is safer if you are having trouble predicting.
Whirlwind can make life hell for a lot of Pokemon, as it can Calm Mind and Whirlwind your Calm Mind's away. With Special Defense, you can even survive a hit or two from Kyogre. Safeguard + Recover owns Blissey. Reflect can help Ho-Oh take physical attacks better. Substitute is also of mention, since Ho-Oh can make 101 HP Substitutes for Deoxys-D and Blissey. SolarBeam has no charge up turns in the sun, but between Sacred Fire and Thunder(bolt), along with the threat of Kyogre or Rayquaza switching in and removing your sunlight, when are you going to use it?
Max Special Attack for the first set, max Attack for the Choice Band set.
For a fast sweeper, you'll want to make it 306 Speed (252 EVs with a +Speed nature). This ties at worst with Groudon, who is very important to outrun. This also often makes it faster than Rayquaza, who also usually has Rock Slide. If you can go before either of them, you can OHKO Groudon, or burn Rayquaza, thus keeping you safe from their Rock Slides.
Tankish versions need more Special Defense and HP, which isn't as bad as it sounds. Ho-Oh is tied for Speed with Deoxys-D, Groudon and Kyogre, making it the slowest uber. It's never going to outspeed Deoxys-A, Mewtwo, almost any Lati@s, and will rarely beat Lugia, no matter how much Speed it has. It's faster than Metagross, too, since Metagross usually just pumps HP and Defense in place of Speed. Since Deoxys-D never has more than 24 or so Speed EVs for Tyranitar (and even that's considered fast), Speed EVs are really just to beat out enemy Ho-Oh, Groudon, Kyogre, and Rayquaza. Rayquaza has a higher base Speed, and will sometimes go Jolly to give those Latias and Latios that only run 307 Speed a scare with Choice Band Hidden Power Bug / Hidden Power Ghost, and you can only tie with Kyogre and Groudon. For this reason, Speed EVs aren't essential. Instead, you could run a 252 HP, 252 Attack, 4 Special Defense spread, as outlined in the Choice Band set, to be able to take more hits from the Pokemon that are faster than you anyway, and just try burning physical attackers on the switch. Of course, if you have Substitute, you need at least 204 HP EVs for 404 HP, allowing 101 HP Substitutes, which are unbreakable by Night Shade and Seismic Toss.
Checks and Counters
Kyogre can come on Sacred Fire most any time, as burning isn't nearly as bad as any other status to it, and absolutely loves SolarBeam, because it makes SolarBeam charge. Latias and Latios resist Fire and Electric, and usually have an Electric move, themselves. Tyranitar resists Sacred Fire, and gets STAB Rock, but doesn't like to be burned. Rayquaza Air Locks Sunny Day and also resists Fire, but unless it's a pure special version, it has to beware of the burn, too. Blissey and Deoxys-D can Paralyze / Toxic non-Safeguarders or Substituters. All of these should watch out for the Choice Band version, especially low defense Blissey, except for Rayquaza, unless Ho-Oh has Hidden Power Rock, in which case even Rayquaza isn't safe.