Press the TV button on the remote first. If the OSSC doesn’t respond by flashing the LED, it is possible the IR receiver is broken, in which case you need to sort that with Bucko, unless you can solder in a new onw yourself.
Ofc before all this also check the remote batteries (red light on remote should blink on button presses). 😉
Otherwise you can try any remote (universal or NEC protocol compatible) and program yourself, there are instructions on the wiki.