Originally Posted by ijgordon

Yes, if you're looking at the nonstop from PDX-LHR that's only operated by BA.
Where it gets more complicated is if you're flying from an AA hub where BOTH AA and BA operate flights -- then you have to decide between (1) buy an AA flight from BA, (2) buy an BA flight from BA, (3) buy an AA flight from AA, and (4) buy a BA flight from AA. And you can mix-and-match the outbound and return to an extent.
But as others have alluded to, the choice of which flight to take (i.e., "operated by") will be dependent upon (a) the importance of EC261 protections, (b) aircraft type/seating configuration, (c) importance of pre-assigning seats which for BA planes you have to either pay for or have OneWorld status for most business class fares and below, and maybe catering/service (BA generally better although in coach/premium economy it's probably pretty close). The choice of who to buy the ticket from will be dependent upon (a) price [including AARP or Chase Visa discount] and (b) mileage/status maximization. Status may play into this as well - if you have status it will be easier to get through to a phone agent for that airline if you need.