With the RFM69 in continuous FSK mode, polling the RSSI register at sufficient rate can be used to acquire OOK signals like on oscilloscope. However when doing so with receiver mode set to OOK, it becomes clear that the RSSI register is used internally for some other purposes:
The stepped behaviour is related to the programmed bit-rate, 2500bps in this case, resulting in bit durations of 400us. The register is used for some filtering and possibly thresholding the signal to generate the signal on the DATA pin. Strangly the RSSI signal trails the DATA signal (I acquire the RSSI first, and the sample the DATA pin!).
However, in OOK mode, the maximum RSSI is still a good indication for strength of the OOK signal, and the behaviour at the noise floor is not impacted compared to FSK reception.