Originally Posted by corporate-wage-slave

It's highly unlikely that the availability was not available on BA.com if it was on Expedia, that was probably a short term IT glitch. We've all had our BA.com experiences, usually leaving it an hour or two, or calling up, resolves that sort of thing. Expedia is looking at the same underlying GDS as BA, both companies just jump through different hoops to get there. Very, very occasionally a TA will sell a fare bucket which has actually sold out, due to latency issues on the systems - in that case BA and Expedia have a communication channel to resolve difficulties. Also occasionally, OTAs use different Points of Sales, that may make a particular fare a bit cheaper, but on simple fares such as LHR-ORK it's unlikely to make a difference. On complex fares it certainly can make a bigger difference, but complex fares are also the ones to go wrong, or just get rescheduled, and it's worth calling up to see if a skilled BA agent can find a way around it. They often can.

BA being cheaper than Aer Lingus, on the other hand, is no surprise, codeshares have different sales bucket and sometimes there are temporary but large glitches. For example an extended family may have just made a booking on Aer Lingus just before the OP and the system may over swing to bump up fares for the next purchaser. Aer Lingus also has its own IT infrastructure and revenue management issues. Usually Aer Lingus would be cheaper, but occasionally not.