The goal of these sets is to hit opponents as hard as possible with varying Speed and power, depending on your choice of items. A Modest nature with Choice Specs maximizes Octillery's power and allows it to outpace most walls in UU, as well. A Timid nature with Choice Scarf, meanwhile, morphs Octillery into an unexpected revenge killer, as it will outspeed all neutral base 100s and maximum Speed base 90s. Water Spout is the key to making both these sets successful, as it will hit extremely hard, even when unboosted. It is also very difficult to switch into, due to its excellent neutral type coverage, so Octillery won't mind being locked into it. Surf is necessary to cover Water Spout when Octillery's HP falls below 63% while Energy Ball and Ice Beam are givens as they provide excellent type coverage.
Thanks to its newest move, Water Spout, Octillery makes a respectable Choice Scarf user in the lower tiers, while a Choice Specs set is a no-brainer. Although it is slower than fellow Water Spout users Blastoise and Wailord, it hits the hardest (even with a Timid nature). It also has the benefit of Energy Ball, which not only hits Water-types for higher damage than Hidden Power, but also has the opportunity to lower Special Defense.
For Choice Specs, a Modest nature is recommended, as Octillery will want all the power it can get. A full-powered Choice Specs-boosted Water Spout is incredibly powerful coming off a 105 base Special Attack, especially when it gets STAB. To put it in perspective, it has a tiny chance to 2HKO Chansey after Stealth Rock damage, an incredible feat for any special attacker. Water Spout's impressive neutral type coverage also makes it very difficult to switch into safely, especially considering Octillery manages to outspeed many of UU's top walls. To avoid sounding like a broken record, Ice Beam and Energy Ball serve the same purpose here that they do in every Octillery set. When your HP falls below 63%, Surf will hit harder than Water Spout, which makes Surf a necessary supplementary STAB attack. It is also a good idea to use Surf if you are forced to take an attack before firing off one of your own, as Octillery's defenses will not keep it above 63% for long.
For Choice Scarf, either nature is acceptable; as per the usual, the decision comes down to what you want to outspeed. Unfortunately, with a Modest nature, Octillery falls one heartbreaking Speed point shy of tying maximum Speed base 80s, which is necessary for it to fully check threats like Blaziken, Kabutops, Medicham, and Venusaur. It still does a decent job of checking these Pokemon, as they often will be running neutral-Speed natures. Timid nature earns the recommended choice, as you not only outspeed all base 80s, but also neutral base 100s and maximum Speed base 90s, a list that includes common variants of Arcanine, Magmortar, Miltank, Nidoking, Leafeon, and Moltres.
With a Choice Scarf, Octillery excels as a revenge killer, as opponents will not expect Octillery to be attacking before sweepers like Moltres and Venusaur. Water Spout also makes it tricky to switch into, as any fragile Pokemon who doesn't resist it will surely take a beating. With its added Speed, Octillery can also rampage through teams in the late game once bulky Waters and speedy revenge killers are taken care of.
Although these sets function quite differently, they benefit from many of the same teammates. Grass-types like Venusaur and Sceptile are recommended to cover Octillery's weaknesses and take out the bulky Waters that stand in its way. Taking out the few Pokemon who can easily take Octillery's attacks is important, as it will allow Octillery to punch holes in the rest of your opponent's Pokemon with ease. A Rapid Spinner is a must-have due to Water Spout's dependency on HP. Entry hazards of your own also help, as Octillery will often force switches. A Pokemon with Thunder Wave, such as Registeel, can help make opposing Pokemon slower than Octillery, which is a huge benefit.