Learn Portuguese (Portugal): Ele está a pintar com o meu pincel.

In Portugal, to say someone is doing something right now, you use the verb estar (to be) plus a plus the infinitive (the original, dictionary form) of the action verb. Here, está (he is) plus a plus pintar (to paint) combines to mean "is painting".

Why is there an o before meu? Doesn't that translate to "the my paintbrush"?

Literally, yes! In European Portuguese, you almost always put a definite article like o or a (the) before possessive words like meu (my). Because pincel is a masculine noun, it uses the masculine article o.

I see meu is used here. Would it change to minha if the person painting was a woman?

No, it stays meu. In Portuguese, possessive words always match the gender of the object being owned, not the person who owns it. Since the paintbrush (pincel) is masculine, it is always o meu pincel regardless of who is holding it.

Is com used for tools the same way it's used for people?

Exactly. Just like the English word "with", com is used both when you are using an object as a tool (like painting com a paintbrush) and when you are accompanying a person.