outdoor为形容词，只能做前置定语。例如 outdoor activities。

outdoors为副词或名词。例如用作副词：go outdoors。He worked outdoors all afternoon.

用作名词：We went for a walk to enjoy the outdoors.

out of doors为习语，= outdoors 例如：The weather was fine for working out of doors.

out of door使用较少。可以忽略。