When combined, “我的天” (wǒ de tiān) literally means “my heaven.” This creates a personal exclamation directed towards the heavens, much like how “Oh my God” is a personal exclamation. It's a way of saying, “Heavens above, I can't believe this!”
The key to understanding “我的天” is the cultural concept of 天 (tiān). In Western culture, “God” often refers to a specific, personified deity from Abrahamic religions. In China, 天 (tiān) is a much broader and more ancient concept. It can refer to the physical sky, but also to a cosmic force, nature, fate, or a divine, impersonal power that governs the universe. It's a concept that predates both Daoism and Buddhism in China. Because of this, “我的天” is significantly less religious than “Oh my God!”. While a devout Christian might feel uncomfortable saying “Oh my God,” virtually no one in China, regardless of their beliefs, would hesitate to use “我的天”. It's a secular exclamation rooted in a deep cultural idea of “Heaven” as a natural, governing force. It's more akin to the English “Goodness gracious!” or “For heaven's sake!” in its secular, universal accessibility.
“我的天” is overwhelmingly informal and is used constantly in casual settings.
Rule of Thumb: If you would say “OMG” in English, you can probably say “我的天” in Chinese. Just avoid using it in formal situations like a business presentation or a serious academic discussion.