Surrounded Regions

Given an m×n board of 'X' and 'O', capture all 'O' regions completely surrounded by 'X' (replace with 'X'). Border-connected 'O's are never captured.

Example: [["X","X","X","X"],["X","O","O","X"],["X","X","O","X"],["X","O","X","X"]] → captured interior O's