Optiboot does not use interrupts. It also does not specifically disable any interrupts, counting on the documented behavior that interrupts are cleared after a RESET.
Optiboot also starts the user sketch by causing a RESET (via the watchdog), so the sketch should always be started with all the uart and interrupt registers in their reset state.
It might be an interesting question as to whether the startup code is "safe" WRT characters buffered in the usb/serial converter during restart...