我的 天 啊

天才 tiancai genius
天使 tianshi angel
天天 tiantian daily, every day
我的天啊 wo de tian a OMG
天气 tianqi weather
天生　tiansheng inborn
天真　tianzhen naive gullible one who believes all they are told
天敌　tiandi sky+tongue+punch=natural enemies

In the ancient world the heavenly emperor above was in his court, which may be why the square 口　was on top of the 大．　口　is sometimes used to depict a square enclosed land area.
Or perhaps the ancient person is so amazed at how vast the sky is his mouth is wide open. In song script the square has become a line so heaven is unitary 一, contains all. If the heavenly is the first world the human is the second world 二　一二人大天　
The song script often as here appears to add ideographic content so we can successfully see and remember the characters in more than one way as part of a related group．