Sunny Day allows Onix to provide utility to its teammates by activating their Chlorophyll or boosting their Fire-type STAB attack. Explosion gains momentum for the team, allowing a Chlorophyll sweeper such as Bellossom to come in without fear of taking a hit. Block, in tandem with STAB Earthquake, gives Onix the capability of beating certain Pokemon such as Swalot one-on-one. Earthquake also prevents Onix from becoming overly passive. These last two moves are fairly situational, however, as Onix's main purpose is to set up the sun and bring in a teammate.
Maximum Speed EVs alongside a Jolly nature and a potential boost from Salac Berry improve Onix's odds of reliably setting up Sunny Day. 144 Attack EVs guarantee the KO with Explosion on Pidgeot, while the remaining 112 EVs go into HP to increase Onix's overall physical bulk so that it can switch into Pidgeot and other Flying-types repeatedly if need be. While Sturdy and Rock Head are both completely useless for Onix, Rock Head could be useful for certain Trace and Skill Swap users such as Porygon and Kecleon, which would suddenly no longer take recoil from Double-Edge. As such, Sturdy is Onix's better ability. Chlorophyll users such as Bellossom, Tangela, and Tropius make great teammates for Onix, as they greatly benefit from the offensive utility it provides. Fire-types such as Flareon can also make use of the sun Onix provides, as it gives a power boost to their STAB attacks. Spikes support from suicide lead Glalie is helpful, not necessarily for Onix but for sun hyper offense teams as a whole.
Other Options
A lead set utilizing Endure over Block allows Onix to more reliably activate its Salac Berry, nearly guaranteeing that it can use Sunny Day. Be aware that Onix loses to a majority of opposing leads such as Glalie, Hitmonchan, Vigoroth, Magby, and Venomoth. Rock Slide can be used over Earthquake as Onix's attacking option, nailing Flying-types such as Murkrow and Pidgeot without having to sacrifice itself with Explosion prematurely. However, Onix has likely already set up Sunny Day by the time the aforementioned Flying-types have switched in, so it's okay if it sacrifices itself. Rock Slide is also less useful than Earthquake when up against Swalot, which can Rest off the damage more easily and KO Onix with Ice Beam or Hidden Power Ground.
Checks and Counters
Opposing Chlorophyll users such as Tropius, Tangela, and Bellossom can all wreck Onix with Solar Beam, put it asleep with Sleep Powder, or set up Swords Dance in Tropius's case. Other Grass-types such as Roselia can also make quick work of Onix, although not quite as reliably. Water-types such as Wailord, Dewgong, Kingler, and Whiscash can douse Onix with Surf or Hydro Pump. Pelipper and Huntail are considerably dangerous, as Pelipper is immune to Earthquake and has immense physical bulk, while Huntail is slower than Onix and can set up Rain Dance after Onix uses Sunny Day to countersweep with Surf. Hitmonchan can deal massive damage with Sky Uppercut and pick off a weakened Onix with Mach Punch. However, Hitmonchan still takes a lot from Earthquake and Explosion. Electric-types such as Plusle and Pikachu carry Hidden Power Grass from time to time, nailing Whiscash, Graveler, Pupitar, and, of course, Onix. Even if they opt to run Hidden Power Ice, Onix will be severely crippled due to its pitifully low Special Defense stat.
