Can I use brass hinges with oak doors, and if not why not?

Yes, but be careful as brass screws are extremely soft and can snap if the screw holes have not been pilot holed. This will also reduce the chance of the screwdriver slipping and 'loosing' the head of the screw.

yes

The short answer is yes - they'll be fine (make sure your hinges have steel washers in them for longevity). Regards J chappell Joinery

Yes. Oak doors are heavy so use three hinges per door with washers.