Hi Phil and welcome to the forum.
So the temperature must not be going much over 200F since you have an o-ring in there? Guessing even a silicone o-ring wouldn't appreciate getting beyond maybe 240F? So running on that...
Don't know the rest of your scoop far as size of these parts, pressure and resulting required clamping force etc. but given the likelihood of less than a 200 degree change in temperature I think this will be a non-issue so long as the bolt you select allows you to apply full seating torque to the bolts.
You don't want to pick a bolt that can't be tightened to the recommended seating torque (derived from it's material, size, and pitch,) without crushing your aluminum pieces. As long as you have enough clamping force and some stretch to your bolts it's not going to lose grip from thermal cycles of approx. 200F. If you go over size with your bolts and can't put a little stretch (seating torque) to your bolts then yes, they would work loose.
Look at aluminum heads on car engines... same thing only different.
Given that you're pulling your nut up on aluminum I'd personally steer clear of a conventional lock washer. Maybe look at a flat washer to protect the aluminum from galling at tightening and use a lock nut.