The idea behind this set is twofold: first of all, Heatran is one of the best Pokémon in the game at forcing switches, due to his nearly universal coverage, great typing, and frequent use of Choice Scarf, so it is not difficult to get a Substitute up. Second, the only Pokemon that are immune to Toxic are Steel-types — easily taken out by Fire Blast and Earth Power — and Poison-types, all of which have either poor Special Defense or a Fire- or Ground-type weakness.
Heatran should be sent out against something that will likely switch out, such as a Celebi or one of the many Steel-types in OU. In consideration of this goal, it is often useful to send Heatran out for the first time after one of your own Pokemon has been KO'd; most players will assume it is holding a Choice Scarf and switch out of the expected speedy attack. Once you have a Substitute up, make your play according to what they switch to. If they for some reason switch in something weak to Fire- or Ground-type attacks, then you attack it, obviously. If they don't, then you are still OK, since most of Heatran's common switch-ins—things like Gyarados, Tyranitar, Salamence, Swampert, and the bulky Water-types—absolutely hate being poisoned. Once you Toxic them and they break your Substitute, you can switch to something to stall out their attacks, and the next time you bring in Heatran your opponent will have at least one fewer counter for it!
Toxic is recommended because it has the greatest coverage of all the options in the last slot, but it isn't that spectacular without a reasonably bulky team to take hits from Pokémon after they have been poisoned. For this reason, several other more specialized options are available if you are running a more offensive team. Explosion is an all-around useful move, carrying the ability to take out Blissey and pretty much anything else your team has trouble with. One of the cool things with Explosion is that almost no one will think to switch to a Ghost-type if you explode with a Substitute up, which just makes it all the more useful (note that a Naive nature should be used with Explosion). A certain Hidden Power can also be selected if you want to be able to quickly take out any one of either Gyarados, Salamence, or Swampert, instead of slowly waiting for Toxic to do the job. Finally, Will-O-Wisp is possibly more useful than Toxic against Gyarados, Salamence, Tyranitar, and some others, but it kills even slower than Toxic (and can even be circumvented via Recover or Roost), and the 75% accuracy is a pretty big turn off.
It may seem strange to use a +Speed nature without a Choice Scarf, but in a metagame where Heatran is as popular as it is, max Speed can be a huge aid. With full Speed, Heatran will attack before +Speed Breloom and Metagross, and before neutral Speed Gyarados, Dragonite, and Heracross; and it will nearly always outspeed the defensive base 100 Speed Pokemon. Any of these could easily OHKO or status Heatran, so getting the jump on them with Substitute or Toxic is often crucial to the outcome of a match.
This Heatran works best when paired with Pokémon who can repeatedly take hits thrown at Heatran and pose a threat to Heatran's counters. Such Pokémon include those commonly found on bulky offense teams.
In order to decide possible partners to Heatran, you have to consider which Pokémon are capable of threatening it. While Substitute greatly reduces the advantage your opponent can gain by switching in an appropriate counter, you will more often than not be forced to switch out after they break your Substitute. Common examples of such Pokémon include Gyarados, who will break your Substitute with Waterfall and then Dragon Dance, and Latias, which can set up multiple Calm Minds with ease due to Heatran's lack of coverage needed to hit it. Both Pokémon are, thankfully, effectively hindered by Toxic, lessening the threat they can pose to your team. Gyarados's best counters which can pose a threat in return are Celebi and Scarfed Pokémon which resist Waterfall or are immune to Earthquake, such as Choice Scarf Latias. Celebi will have trouble retaliating against boosted Ice Fangs, but can effectively stall out Gyarados with Recover until the effects of Toxic finish it off; however, you must be careful to avoid getting hit by Bounce, should Gyarados be carrying the move. Calm Mind Celebi makes a great choice for a bulky offense team, as it not only counters specific threats, but can also pose as a threat of its own after a few boosts; the Tinkerbell set is also a worthwhile option if your team relies on slower sweepers who appreciate paralyzed opponents. If using a Scarfed Pokémon as your check to Gyarados, it should come equipped with either an Electric-type attack or an attack with a high Base Power; in Latias's case, it has access to both Thunderbolt and Draco Meteor to hit Gyarados with. You should also make sure to watch out for possible Pursuit users your opponent might be carrying, especially Scizor and Tyranitar, which take little damage from attacks likely to be thrown at Gyarados. Latias can be handled by most Steel-types outside of Heatran, particularly those with a high Attack stat, such as Metagross, Scizor, and Jirachi. Metagross can either set up Agility or fire STAB Meteor Mashes to keep Latias at bay. Jirachi can either use Iron Head repeatedly if running Choice Scarf, or use a combination of Body Slam and Iron Head to weaken Latias. Scizor has both U-turn and Pursuit to murder Latias with. As always, make sure to watch out for Magnezone, a likely partner whose sole role is to take out Steel-types which threaten Latias. On an added note, some Latias pack Refresh, so don't over-depend on Toxic to take out Latias.
This Heatran set will also have trouble against stall, particularly because Spikes and Stealth Rock will limit the number of times it can switch in. To make it worse, Gyarados with Rest, and even Blissey, two Pokémon commonly found on stall teams, will wall this set with ease, as they are unhindered by your attempts to poison them. As such, an effective mixed sweeper is recommend as a partner to Heatran, particularly those specifically designed to weaken stall teams. Classic MixMence, Salamence's most effective wall-breaking set, makes a fantastic partner to Heatran, especially since they cover each other's weaknesses extremely well; Heatran takes Ice- and Dragon-type attacks thrown at Salamence, while it can comfortably take Water-type attacks along with an immunity to Ground-type attacks aimed at Heatran. Be prepared to rely on prediction, such as using Brick Break on a Blissey switch-in, when using MixMence, as any expert stall player will normally be prepared for common wall breakers. There are also some other worthwhile options you can consider as well. A defensive Starmie can come in on Water-type attacks aimed at Heatran with ease and take the opportunity to use Rapid Spin to eliminate any present entry hazards. Running a defensive Starmie also allows you to have a suitable counter to Gyarados, one of the biggest threats to Heatran. Additionally, Starmie is one of the few Spinners in the game that can actually threaten Rotom-A with its STAB attack of choice and decent Special Attack stat.
If you want Heatran to run another attack instead of Toxic, such as the listed Explosion, it is often a good idea to pair Heatran with a user of Toxic Spikes. However, Toxic Spikes are unable to affect non-grounded Pokémon, so Toxic should not be completely dismissed, even with Toxic Spikes support. As mentioned before, Roserade is probably your best bet in laying down Toxic Spikes, but Smeargle is also a good option if you want something to lay down Stealth Rock as well. The fourth slot is mainly used to hit Pokemon that will attempt to wall you, so if you don't like the drawbacks to using Explosion, then Dragon Pulse and Hidden Power Electric are both decent options as well, but it should be noted that neither of the two moves will be even close to being as helpful and reliable as Explosion would normally be. Also, when using either of these attacks, it is highly recommended that you pack a teammate capable of countering the Pokemon you are able to come in and set-up on the attack of your choice. For example, Gyarados will be capable of setting up on Heatran running Dragon Pulse, so running a Celebi or Starmie (or a Scarfed revenge killer such as Latias) would be almost necessary in countering Gyarados.
An additional option that this Heatran can utilize is to run Roar over the given options for the fourth slot. Roar is best used on a team which revolves around spreading damage through entry hazards, often normally regarded as a "stall-based balanced" team. Such a team utilizes heavy hitters with valuable resistances, like Heatran itself, to force switches and spread even more damage with the addition of Roar into the set. In this case, Heatran's primary partner is one who is capable of at least laying down either Spikes or Stealth Rock. In the case of Spikes, Smeargle, Forretress, and Skarmory are all decent options for reliably laying down the essential entry hazards. Smeargle is best used as a lead, where it can use Spore to immobilize the opposing lead (unless it runs Taunt or is not Choice Scarf Jirachi) and nearly always lay down at least two layers of Spikes. Skarmory and Forretress are capable on coming in on many of the attacking types found in OU due to their valuable Steel-typing, giving them ample opportunity to lay down Spikes. The choice between Skarmory and Forretress is slightly arbitrary, as both have their advantages; Skarmory has access to Roost and Whirlwind, while Forretress can provide the team with Rapid Spin capabilities. As for Stealth Rock, there are plenty of viable users to choose from, but Swampert will more than likely be your best choice, as it pairs well with Heatran type-wise and also has access to Roar itself. Next, you will also want to pair Heatran with other Pokemon capable of taking advantage of entry hazards. Namely, you'll want to at least run a Ghost-type, such as Rotom-A, to prevent your entry hazards from being spun away. Apart from that, Latias with Roar is an excellent teammate for Heatran, as both cover each other's weaknesses relatively well and can each beat each other's counters.
As always, Wish support can be used in aiding the overall longevity of Heatran, especially with Substitute taking away 25% of Heatran's health each time it is used. Vaporeon is the best option for this task, as it can easily switch in on Water-type attacks normally aimed at Heatran and even regain its HP in the process, thanks to Water Absorb.
