FWIW, I received a few J to F op-ups last year (plus one WT+ to J op-up), and I always use OLCI (well, at least on long hauls).

Oddly, all of my op-ups were going to London. No op-ups from London for me.

Mind you, I'm not complaining at all about a lack of op-ups frm London ... (especially as the J tickets were MFU from WT+). I just wonder if perhaps the selection procedure varies by departure point (departing London they might look at your gold score more closely, where I'm sure mine is relatively low compared to years past).

Each of the J to F op-ups, when I used OLCI (at around the 24 hour window), I checked in to J seat, and was given an F boarding pass at the airport check-in desk.