OK button is one of those reserved and, even if OBH lets you a great freedom at changing keys' functions (Setup/User interface/Button setup), OK won't be available to be customised.
But the question is: are you really, ,really sure you want to change such an important key just not to simply use UP or DOWN to open channel list? What for?
OK is a basis key to recall infobar and 2nd infobar.
OBH has got, in this regard, the very comfortable chance of distinguish the 1st Infobar timeout from the 2nd. You may take advantage of this to finally have an Infobar that appears and disappears at your will, just by pressing OK. How?
Just setting whatever timeouts (in seconds) you like for the 1st Infobar (let's say 3 secs) and NO timeout for the 2nd Infobar.
This way, pressing OK twice, you'll have a fix (2nd) Infobar until you press again OK, and that's is very, very comfortable under many circumstances.
So, even if you manage to change the keymap, you'll loose a lot of control on your infos.
I wouldn't change this just not to press UP or DOWN, instead.
But this is obviously subjective.