building a minimalistic rf circuit (as simple as possible just for OOK)
I'm going to answer the "minimalistic rf circuit" bit. As the comments note, 125kHz is a tall order. I'll assume that 433MHz, being unlicensed, practical and popular, will suffice.

A big caveat up front: RF is not simple! I'm assuming this is a learning exercise for you. In that case, carry on! If instead you just want to use rf, then just buy one of the many, many dirt cheap RF transmitter/receiver modules.

With that out of the way, here is the simplest RF transmitter circuit I know of:







simulate this circuit &ndash; Schematic created using CircuitLab

V2 is your OOK signal, V1 is your power supply (approx 5V), L1 is your antenna and the "SAW" component is a 1-Port SAW Resonator. The critical values are C2, C3 and L1 - the values I've provided are just starting points. They are highly unlikely to work for you! They depend a great deal on your layout, PCB materials and component variation. It will take some experimentation to get it right. Just make sure the series combination of C2, C3 and L1 resonate at the frequency you want (433Mhz is a good target).

L1 will be the most difficult to control - try a coil of wire whose dimensions you can adjust as you go. The rest of the components should be SMD and mounted close together.