Omanyte - High Tier
-Availability: Omanyte is accessible as soon as you have the ability to Surf, which requires the defeat of Koga. Omanyte comes at a strong Lv30, 10 levels away from evolution, and can defeat the local gym rather quickly for free experience.
-Stats: Omanyte possesses good Defense and Special, allowing it to have excellent bulk at both ends of the offensive spectrum. Said Special also gives him great offense as his STAB Surfs are ridiculously powerful, and he can use Ice Beam/Blizzard as auxiliary coverage. Omanyte has average speed but it is enough for most in-game purposes.
-Movepool: Omanyte has instant access to one of the best moves in the game in Surf, and is an investment well worth making since Omanyte with Surf is capable of steamrolling Blaine's entire gym at base level. Omanyte can also use Ice Beam / Blizzard to great effect, especially against enemies such as Lance.
-Power: Instant access to 95 BP STAB does wonders for Omanyte, especially when it makes training a breeze. Omanyte has instant wins over Blaine and RB Giovanni/Lance, and can help out significantly in most other battles due to the sheer power of STAB Surf.
-Type: Omanyte's Rock-typing is a double-edged sword. It does not bring any STAB to the table, not even Rock Slide. Rather, it brings to the table a Fighting and Ground weakness and compounds the Grass weakness of the Water-typing. However, Rock also gives it a nice resistance to Normal to complement its excellent defense. Omastar's coverage is generally limited to Surf/Blizzard or Ice Beam, but Omastar is strong enough that it usually ends up spamming these most of the time.
-Matchups:
Erika: Chances are you will not have Omanyte around for this battle, but regardless it's best not to use Omanyte here, even if you have taught Ice Beam or Blizzard. This is because Erika's Pokemon will outrun and hit hard with Razor Leaf/Mega Drain.
Sabrina: Omastar is a decent choice for this battle, although don't expect it to contribute significantly, since Sabrina's Pokemon have equally good Specials.
Blaine: Omanyte can literally walk in here at base level and steamroll everything. Due to a resistance to Normal/Fire, Omanyte tanks almost everything here with ease even when underlevelled and retaliates hard with Surf.
Giovanni: Omastar will have a good time here in RB due to everything being weak to Surf. In Yellow, Giovanni packs Earthquake Dugtrio and Thunder Nidos that make using Omastar a risky prospect. He's still one of the best candidates to kill Rhydon, if not for free EXP.
Rival [Pre-Victory Road]: It wins Pidgeot and Exeggcute with Blizzard, as well as all the fire types and Rhydon with Surf. It's stalemated against Gyarados/Alakazam, while Blastoise and Venusaur can beat it.
Lorelei: Not a battle that Omastar performs well in. Her stuff either resists Surf, Ice Beam/Blizzard, or both, not including that all of them have relatively good special.
Bruno: A cakewalk. Omastar can survive one of Hitmonlee's attacks and retaliate with Surf, Hitmonchan and Onix are both jokes, and Machamp can be handled if Omastar enters with full HP.
Agatha: The only thing you really need to watch out here is Mega Drain/Psychic Gengar, otherwise Omastar can tank everything else easily due to them being physical attacks.
Lance: RB Lance is easily taken care of by Omastar with the support of an electric-type to remove Gyarados. For Yellow, it's slightly trickier since Omastar has both Thunderbolt Dragonair and Thunder Dragonite to contend with.
Rival [Champion]: Almost the same deal as the other rival battle, i.e. bad against Blastoise/Venusaur/Exeggutor, wins Pidgeot/Rhydon/Fire-type, stalemates on Alakazam/Gyarados.
AC: Omanyte's main asset is being a late-game water: it's also one of the strongest at that. Rock-typing makes playing Omastar slightly more interesting than the others, but overall it's still one of those generic water-types.