Assuming a radically offensive Heatran:

Heatran @Life Orb
252 SpAtk / 252 Speed - Timid nature
Fire Blast / Hidden Power [Ice / Grass] / Earth Power / anything

And using sets provided by the Pokemon Showdown Calculator, Heatran could perform as follows (no terrain / weather / stat boosts):

Donphan @Leftovers ("UU Offensive Utility")
252 HP / 24 Atk / 148 SpDef / 48 Speed - Adamant nature

• Heatran outspeeds Donphan and deals 74.4-87.7% damage with Fire Blast -> guaranteed 2HKO if the switch-in is predicted correctly
• Donphan OHKOs Heatran with Earthquake

Hippowdown @Leftovers ("OU Specially Defensive")
252 HP / 252 SpDef - Careful nature

• Heatran outspeeds Hippowdown and deals 49.5-58.5% damage with Fire Blast -> potential 2HKO if the switch-in is predicted correctly and both attacks roll high damage (due to Hippowdon's Leftovers)
• Hippowdon OHKOs Heatran with Earthquake

Garchomp @Rocky Helmet ("OU TankChomp")
252 HP / 252 Speed - Jolly nature

• Garchomp outspeeds Heatran and OHKOs with Earthquake
• If the switch-in is predicted correctly, Heatran deals 92.8-110.2% with Hidden Power [Ice], which is almost a guaranteed OHKO with Stealth Rock
• More offensive sets with no HP investment are a guaranteed OHKO if the switch-in is predicted correctly

Torterra (doesn't really matter which set)

• Heatran outspeeds Torterra, and even with maxed HP and SpDef and a +SpDef nature, Fire Blast deals 94.9-112.9%. Without the +SpDef nature, it's a guaranteed OHKO
• However, if Torterra gets to attack, Earthquake will OHKO Heatran

Quagsire @Leftovers ("1v1 Stall")
252 HP / 160 Def / 96 SpDef - Impish nature

• Heatran outspeeds Quagsire and deals 66.8-79.2% with Earth Power, a guaranteed 2HKO if the switch-in is predicted correctly
• This Quagsire set usually has Bulldoze, which only deals 66.8-79.2% damage, but an Earthquake would OHKO
• With Hidden Power [Grass], Heatran OHKOs. Even max SpDef EVs would only lower the damage to 89.8-106.8%, which is still a potential OHKO

Gastrodon @Leftovers ("OU Specially Defensive")
252 HP / 252 SpDef - Calm nature

• Heatran outspeeds Gastrodon and deals 65.9-78.1% with Hidden Power [Grass], which would be a guaranteed 2HKO, but Earth Power (if HP [Ice] is chosen) only deals 24.4-28.8%
• This Gastrodon set usually has Chilling Water as its only offensive move, which only deals 30.3-35.9% damage. Were it to run Earth Power (or Earthquake with a Careful nature), it could OHKO Heatran.

Mamoswine @Focus Sash ("OU Stealth Rock")
252 Attack / 252 Speed - Jolly nature

• Mamoswine outspeeds Heatran and OHKOs with Earthquake
• If Heatran predicts the switch-in correctly, it can OHKO with Fire Blast

Sandy Shocks @Heavy-Duty Boots ("OU Offensive Pivot")
252 SpAtk / 252 Speed - Modest nature

• Sandy Shocks outspeeds Heatran and OHKOs with Earth Power
• If Heatran predicts the switch-in correctly, Fire Blast will only deal 85.5-101.6%, which makes an OHKO unlikely

Iron Treads @Leftovers ("Uber Specially Defensive")
252 HP / 252 SpDef - Careful nature

• Heatran outspeeds Iron Treads and OHKOs with Fire Blast
• If Iron Treads has maxed out speed (e.g. the "OU Offensive" set), it will outspeed Heatran and OHKO with Earthquake

Ursaluna @Flame Orb ("OU Swords Dance")
116 HP / 252 Atack / 140 Speed - Adamant nature - ability: Guts

• Heatran outspeeds Ursaluna and deals 65.5-77.4% with Fire Blast, a guaranteed 2HKO if the switch-in is predicted correctly
• Ursaluna can OHKO Heatran with Headlong Rush, and if Guts is activated, even an unboosted Facade might OHKO with 87.6-103% damage

Clodsire @Leftovers ("OU Defensive")
248 HP / 252 SpDef / probably 8 Def I forgot - Careful nature

• Heatran outspeeds Clodsire but only deals 39.9-47% with Earth Power
• Cladsire can OHKO Heatran with Earthquake