[gimmepie]

If you're looking for dark/psychic/ghost pokemon (regardless of generation) these are some to consider.

Sableye - No weaknesses

Spiritomb - Sableye but better

Gallade - Negates weakness to dark types. Good move pool.

Scrafty - Immune to Psychic moves which is helpful for a fighting type.

Gengar - They're fast and have levitate to negate their ground-type weakness. But are weak to both psychic and dark.

Sigilyph - Good special attack and reasonable speed. Good move pool.

Tyranitar - It's got awesome stats. Enough said. Good against electric types (since you started with Oshawott)

Krookodile - Kind of like a mini-tyranitar. Easier to obtain. Can have moxie which is a great ability.