select airline or route on ticket?
 

If as part of your itinerary a BCN-LHR segment is required. Would you get it ticketed by particular airline or just the route as it is served by more than one airline? In the event one is sold out could you use another?

Unless left open it would be ticketed on a particular airline / date / time. But there's nothing to prevent you switching airline provided it doesn't involve re-routing [which might mean re-issue of the ticket and USD 100 after initial issue]

Even with open dated tickets, I am pretty sure that you still need to specify a carrier for each segment.

IF, however, BA and IB codeshare on this route, you might be able to get a seat on the other carrier's flight by booking it under the codeshare # of the carrier listed on the ticket.

As far as I've ever seen, a carrier must be specified but there's no reason you can't have the ticket endorsed over to another OW carrier, even at the airport. I've had BA cheerfully offer to take an AA A-class ticket LHR-LAX.

In general there is no problem at all using Open tickets on valid carriers other than the one specified on the ticket. The ticket is clearly endorsed "Valid on BA/AA/CX/QF/IB/LA/etc only" and 9 times out of ten you can just present the ticket at check-in to the airline that has your reservation.

The only time I had to get a sticker endorsement (from AY to BA) was at HEL where AY were doing a pretty poor job in general of demonstrating that they were part of OneWorld (clueless on lounge access with a CX Diamond card, etc.)

That surprises me, I've always found AY to be one of the better OW carriers at recognising status. Only QF could produce an Emerald level card which looks as if it ought to be lower down the pecking order than Gold...