I've heard about it when someone was talking about drum mic positions, but I'm not sure. Is it something like overhead mic position?
OTK? On the kick. Not in front or inside, but actually 'on the kick'. Mounted on the rim of the bass drum.
At least one person in a podcast defines it as “over the kick”. This looks like an automatic transcription that has no punctuation and I have quoted it exactly (emphasis mine):
anyway okay so here's a mic that I was talking about earlier the otk over the kick Mike so this Mike is literally over the kick drum like maybe an inch or two over the kick drum either facing down at the kick drum facing the snare sometimes facing the floor tom sometimes it's more at the rim of the kick drum have to experiment the place in a little bit I tend to like somewhere around the middle of the kick drum facing the snare kind of equidistant between like floor tom rack Tom and snare sort of where those sort of have a middle point somewhere around the center of the kick drum underneath the ride cymbal somewhere in there and this is what that might sounds like this is a really interesting Mike
If you look at the table and picture closely on this site:
you’ll see that there is a mic that looks like the OTK mic boomed in from the hi hat side over the batter side rim of the kick.
This video goes with that site and the OTK mic is pointed out at around 6:10:
I suspect this is partly a batter side kick mic for picking up articulation and/or kind of a crotch mic that gets a nice central image of the kit.