interjectional phrase is correct because Oh my God! means a group of words used as an exclamation to show surprise, shock, fear, or strong feeling.

This idiom or phrase is useful because it helps students understand the intended meaning beyond the individual words.

Examples: Oh my God! I forgot my homework. Oh my God! That mountain is beautiful.

The expression contains several words and functions as an exclamation, so interjectional phrase is the best answer.