This game was designed by Mark Steere; see his website for other of his games.
Detailed rules for Oust are
here (pdf file).
Briefly, the goal of the game is to eliminate the opponent's stones.
Stones are placed on the board, never moved, but can be captured.
Captures are effected by placing a new stone adjacent to one's own existing stones, thus creating a larger group: any group of the opponent's stones contiguous with that newly enlarged group are removed from the board, but such a move can only be made if all of the opponents affected groups are smaller in size than the enlarged group. After a capture a player gets an extra move, which is realized here by requiring the opponent to pass. Noncapturing moves are placements which do not touch any of one's own stones.