In the group 'A, B(,) and C' the comma before 'and' is optional. It is much less common to have 'or' instead of 'and', so people are less sure about how to write 'A, B(,) or C', but my opinion is that the same conventions apply. It is not important whether you write 'A, B, and/or C' (with comma) or 'A, B and/or C' (without comma). There is no rule of grammar - there is no rule of English - about this. It is your choice.
But there is a very strong convention that we do not put a comma between a subject and its verb, even if the subject is long and complex. If 'A, B(,) or C' is the subject, you write 'A, B(,) or C does X', not 'A, B(,) or C, does X'.