Onix is very cool for UU as it's the only Rock / Ground type there. It can come into any Electric as long as it avoids Electabuzz's Psychic. It just doesn't have the stats for anything major though; don't even try using it in OU, where it is completely outclassed by Golem.
Get Onix in against an Electric attack (from Magneton or something) and force a switch to a Pokemon that's slower than it, preferably a paralyzed Pokémon. Bind it repeatedly and finish off it with Rock Slide or Earthquake when it's in KO range. Explode when Onix cannot go anymore. Beware: Onix is really weak, and it doesn't even OHKO Alakazam with Explosion.
Body Slam can paralyze stuff and Screech lowers their Defense. The time you use Screech is probably the time you should be using Bind, though.
Checks and Counters
The following standards have no problem at all with Onix: Alakazam, Venusaur, Exeggutor, Starmie, Lapras, Chansey (won't switch in, but can beat Onix with Ice Beam in the long run), Golem, Rhydon (can both take a hit and then deal some damage with Earthquake), Gengar (if your Gengar has Mega Drain or Psychic), Slowbro, and basically any Water- or Grass-type Pokémon.
In UU, you'll mostly want to use Water- and Grass-types. Blastoise, Vileplume, Tangela, and Vaporeon shut it down quite effectively, and while Tentacruel can't switch into Earthquake, it can OHKO Onix. The same goes for Seadra, Kabutops, and Omastar. Beware of Mega Drain on random things, as any special attack will hurt Onix.