Figued it out after much experimenting. Apparently the OT doesn’t respond to Bank Change in a traditional way. Instead what bank change does is change between bank groups ABCDEFGH and IJKLMNOP. Bank Change value 0 puts you into the first group which stores 128 patterns which you can then navigate with Program Change messages. Any other Bank Change value puts you in the second group. A Bank Change command always has to be followed by a PC for it to actually change the bank.
Once you are in the second bank group any Pattern Change (PC) will take place within that second group until you change back to the first one again via Bank Change. Unfortunately the same does not go for a manual Bank Change into the second group. Any PC will make you jump back into the first group again. Weird.
Would have saved me an entire afternoon if there would have been any mention of this unconventional behaviour in the manual. oh well…