from collections import deque

def surrounded_regions(board):
    if not board or not board[0]:
        return board
    rows, cols = len(board), len(board[0])
    queue = deque()
    def mark(r, c):
        if 0 <= r < rows and 0 <= c < cols and board[r][c] == 'O':
            board[r][c] = 'S'
            queue.append((r, c))
    for r in range(rows):
        mark(r, 0)
        mark(r, cols - 1)
    for c in range(cols):
        mark(0, c)
        mark(rows - 1, c)
    while queue:
        r, c = queue.popleft()
        for dr, dc in ((1,0),(-1,0),(0,1),(0,-1)):
            mark(r + dr, c + dc)
    for r in range(rows):
        for c in range(cols):
            if board[r][c] == 'O':
                board[r][c] = 'X'
            elif board[r][c] == 'S':
                board[r][c] = 'O'
    return board