I am also curious to know if an OID for airtime utilization exists.
We currently use the command "get airtime wifiX" (where X=0 for 2.4GHz and X=1 for 5GHz) per AP. This command gives raw counters for Total, Busy (interference/PHY errors), Rx and Tx for the moment you issue the command.
For example, on an APs you can have this:
rkscli: get airtime wifi0
Total: 430 1AE
Busy: 35 23
Tx: 265 109
Rx: 130 82