What part of speech is oh my in the sentence, oh my, how did you get stuck in your chair

The part of speech which of "oh my" is called an interjection.

In English grammar, interjections are used to express emotions such as surprise, joy, anger and disgust etc.

In this sentence, "oh my" is used to express surprise. The speaker, or writer is surprised and wondering how someone managed to get stuck in their chair.