Q. If the word “god” is capitalized only when it is a proper name, why would you capitalize it in the expression “Oh my god!” unless you know that the speaker is referring to the specific deity worshipped by Christians and other monotheists? Does Chicago style uppercase or lowercase “Oh my god!”?
A. In general, when “god” is used nonliterally (as in your example), or when the reference is to plural “gods” (or to one god among many), lowercase g is the better choice; as your question suggests, a capital G is normally reserved for literal references to the supreme being (or Supreme Being, when referring to a specific God) worshiped according to any of a number of monotheistic religions. But religion is as varied as it is personal; some authors will prefer to capitalize “god” even in apparently nonliteral references. And some may prefer plural “Gods.” Editors should therefore try to confirm an author’s preference before making any wholesale changes. For some additional considerations, see CMOS 8.91.