Who activates the fasten seat belt warning on commercial flights? Who activates (and de-activates) the "fasten seat belt" warning on commercial flights? I would guess it is either the pilot in command (normally the captain) or the first officer (co-pilot).

If the answer is airline specific, then answer for American Airlines.
It's P1's decision, and P2's hand that moves the switch.
But, there's a really nice position of the switch, "Auto", which switches them on/off automatically based on some rules (which are usually passing through 10,000ft). Using the "Auto" position means that you can't forget them!