offensive Mamoswine is mine
Type: Ice / Ground
Base Stats: 110 HP / 130 Atk / 80 Def / 70 SpA / 60 SpD / 80 Spe
Abilities: Oblivious / Snow Cloak
Mamoswine is can be an effective physical attacker once the things that wall it are removed. It has all of the tools: an off-the-charts Attack stat, a great STAB combination, and a powerful priority attack, Ice Shard no less. Its Earthquake steamrolls through all but the bulkiest of resists, holding the crown of the most powerful Ground-type attack commonly found in OU. An Ice Shard coming from it is one of the most potent attacks found in OU, allowing it to revenge kill common Dragon-types, such as Dragonite and Flygon. It also has access to powerful coverage moves like Stone Edge and Superpower, the latter putting a dent in the only common Pokemon that resists Mamoswine's STABs, Bronzong. Stealth Rock and Endeavor allow it to function as an excellent lead, with the Endeavor + Ice Shard combination being a great way to ensure that opposing leads faint. Its typing makes it immune to both sandstorm and hail and takes away the curse of being weak to Stealth Rock that is normally attributed to Ice-types. To top all of this off, Mamoswine has a useful ability in Snow Cloak that allows it to abuse hail with a 20% boost to its evasion.
EDIT: taking defensive Blissey because I can
Base Stats: 255 HP / 10 Atk / 10 Def / 75 SpA / 135 SpD / 55 Spe
Abilities: Natural Cure / Serene Grace
Blissey is and has always been the premier special wall of OU, barring Chansey in RBY. Its gigantic HP and Special Defense stats allow it to wall most special attackers in existence that lack Explosion, the few that can beat it including SubSplit Gengar and CroCune. With 255 base HP, its absolutely horrific Defense stat is alleviated, allowing it to take some physical hits from some walls and cripple them with Toxic. It has all of the support options it could ever need, including a recovery move, status moves, Wish, and Aromatherapy. These allow it to take on many roles, including a Wish passer to keep the rest of the team healthy, a cleric to keep the rest of the team free of status, or even a paralysis spreader to allow some slower Pokemon to shine. It can even setup Stealth Rock if desired, finding many opportunities throughout the match against numerous special attackers that it walls oh-so-well. Yet another plus is its usable Special Attack stat and expansive special movepool, including notable options like Flamethrower and Ice Beam, the former for Steel-types and the latter for Dragon- and Flying-types. With all of this, it even gets the ability Natural Cure to heal off all status, including the Toxic that most every wall deathly fears. Blissey can also run some Speed EVs to outspeed certain threats like Scizor and KO with the appropriate move, in Scizor's case Fire Blast, due to its access to many coverage moves, the aforementioned Fire Blast, Ice Beam, and Thunderbolt being the most notable.
EDIT2: offensive Empoleon and offensive Heatran are mine
Type: Water / Steel
Base Stats: 84 HP / 86 Atk / 88 Def / 111 SpA / 101 SpD / 60 Spe
Empoleon has quite a few movesets it can run, though many are not all that common. Regardless, it has many different options that it can run, each with a different set of checks and counters. Its most common set features Substitute, Agility, 2 attacks, and a Petaya Berry, aiming to use Substitute whilst boosting its low Speed in order to reach Petaya Berry range, from where it can use its likely boosted Speed and +1 Special Attack from the Petaya Berry to sweep unhindered once certain threats are out of the way. In addition, once Empoleon has used Substitute to a range where Petaya Berry is activated, it is also in Torrent range, which makes its STAB Water-type attacks even more powerful. Empoleon also has access to Stealth Rock and Aqua Jet to function as a great lead, Stealth Rock near-guaranteed to be setup with Aqua Jet to finish off leads down to their Focus Sash. It can even utilize Choice Specs effectively with its solid Special Attack stat and access to great moves like Hydro Pump, Ice Beam, and Grass Knot to get great coverage against the majority of Pokemon.
Type: Fire / Steel
Base Stats: 91 HP / 90 Atk / 106 Def / 130 SpA / 106 SpD / 77 Spe
Ability: Flash Fire
Heatran is the face of DPP OU, consistently one of the most-used Pokemon in the tier, and it doesn't take much digging to figure out why. Its typing gives it the resistances of a Steel-type and the coverage of a Fire-type, along with a neutrality to the ever-so-common Stealth Rock instead of the weakness that plagues most Fire-types. Its main set is a Stealth Rock lead with Shuca Berry, allowing it to setup Stealth Rock against the likes of Metagross and Flygon. That set may also use Passho Berry to setup Stealth Rock against any lead carrying a Water-type attack, such as Empoleon or Gyarados. It can also don a Choice Scarf to revenge kill many top threats, taking away any issues Heatran may have with Speed and utilizing its excellent coverage, STAB Fire-type moves, and Explosion to take down special walls by sacrificing itself. However, those are only a few of Heatran's many options. Heatran can also use Torment to annoy opponents to great effect, as well as Magma Storm to function as a trapper. Substitute, Choice Specs, and Life Orb are all just as threatening offensive options, the former protecting Heatran from faster Pokemon with STAB Water-, Fighting-, and Ground-type attacks. It can also utilize Metal Sound to allow Fire Blast and its coverage moves to hit even resists and special walls hard, particularly allowing Heatran to defeat Blissey without fainting itself in the process. It even has access to Taunt, which prevents paralysis and sleep from crippling Heatran. Heatran can literally fill any role and excel at it, so beware whenever it leaves its Poke Ball, lest it tear apart one's team.