Hermes1964, I checked the oneworld site and it shows Iberia uses two lounges – an “Executive Club Lounge” and an “SEA” lounge. I am not sure which you used but given that the former is badged for IB and BA, and of course BA’s frequent flyer programme is called the Executive Club, I am guessing that the ECL is a BA lounge and that you went to the SEA lounge, which sounds a bit sub-par. Why they gave you that lounge and not the other I am not sure – perhaps the other lounge is reserved for top-tier frequent fliers (I am only guessing)
However, if you are flying a premium cabin on a oneworld airline (such as IB), you can use any lounge for the equivalent class offered by a oneworld airline at that airport – so you could have used the ECL (assuming it is a business class lounge rather than first class) or, indeed, AA’s Admirals Club or LAN’s TAM lounge.
oneworld don’t do a very good job of publicising this, but the ability to use any oneworld airline lounge of the equivalent class is definitely there, and equally definitely worth bearing in mind!