Should I use a comma before 'or' if it menas 'in other words' ?

Answer 1

Yes, if it's a compound sentence. Since you're using it to introduce a complete alternative version of the sentence, with its own subject, verb, and predicate, you should put a comma before 'or', or you can think of the sequence "comma, or" as being equivalent to a semicolon.

Answer 2

Yes. Example: "Evolution, or the theory of common descent, was introduced by Darwin."

Answer 3

Yes. If you are giving the meaning or definition of a word, the comma makes it clear that you are doing that.