Here is the full list of all 20 Othello achievements.
Beat the AI on "Beginner"
Beat the AI on "Easy"
Beat the AI on "Medium"
Beat the AI on "Hard"
Beat the AI on "Harder"
Beat the AI on "Hardest"
Win as White at least 19.84 times.
Tie a game
Flip out.
Win your first online match
Win 3 games in a row, online
Win 5 games in a row, online
Win 10 games in a row, online
Win 15 games in a row, online
Lose an online game
Win your first game against a human opponent
Beat a player who has the 'Insane' achievement
Flip six tiles in one move.
Flip twelve tiles in one move.
Flip eighteen tiles in one move.