On CO, OLCI is available from (and to) most international destinations (all except: Dublin, Ireland (DUB); Grand Cayman Island (GCM); Geneva, Switzerland (GVA); Shannon, Ireland (SNN) and Zurich, Switzerland (ZRH). Other exclusions: paper tickets, flights with 5+ segments, or parties with 12+ people.

The key with CO is that you need to scan your passport ONCE at an eServiceCenter (CO's version of the easy-chicken kiosk), and as long as your passport is still valid, that information is already in the system for future OLCIs. It does kick you out on occasion to see an agent (e.g. to DEL, an agent needs to verify your visa, etc.).

With UA, it's US-only, outbound. [I may be mistaken, but do recall seeing something for Japnese MP members about OLCI from NRT].

I've done OLCI internationally on SQ, BA, US and AF, including from/to the US, so it is done by other many other airlines. I think AA and DL do not do international OLCI (Puerto Rico and USVI not counting), unless they've added this facility recently. The way it works differes by airline; in many cases someone needs to verify your passport but you're checked in, with seat assignments, with standby upgrade priority, with OLCI bonuses (to the extent there are any left out there), etc., etc. I think CO's approach is the most clever.

Hence I don't buy the musings on this thread that it's because of either US or non-US HLS, DHS, TSA, APIS or whatever alphabet soup you throw at this.