Is comma before “and” mandatory or not?

Question

I only put a comma, sometimes, not always, when I feel like the successive clause could stand in its own sentence.

I only like to talk about the weather, and to go onto a monologue about the great person that I am.

but sometimes I also write it like this:

I only like to talk about the weather and go onto a monologue about the great person that I am.

So is it a mandatory thing. I believe there was a similar thread about comma before and, but the answer seemed to mention the Oxford style, which is when we put a comma between various listed elements and not clauses.

Answer 1

The comma before 'and' is not necessary because the clauses aren't independent. They're both infinitive clauses based on "I like." I think the answer to this question is clearer when you use a simpler example:

I like to eat and [to] drink.

Examples 13 and 14 from this link: https://owl.purdue.edu/owl/general_writing/punctuation/commas/extended_rules_for_commas.html

are helpful. The basic explanation is that between two verbs or verb phrases in a compound predicate, commas are unnecessary, and the same is true between two nouns, noun phrases, or noun clauses in a compound subject or compound object.

Answer 2

The comma changes the meaning (although you also dropped the "to" between "and" and "go", which reinforces the change in meaning). Your first sentence means "Talking about the weather is something I like, and going onto a monologue is another thing I like." Your second sentence means "Talking about the weather and then going onto a monologue together make up something I like.

Answer 3

It's known as an Oxford or a Serial Comma.

Oxford itself says that...

The 'Oxford comma' is an optional comma before the word 'and' at the end of a list

It is used for clarification i.e. to avoid any ambiguity.

One of my favorite ways to make my students understand it in a hilarious way is -