Pucci, I think that depends on the airline ...

I regularly travel with AA and CX and have never had an issue accessing one of their lounges. Further, AA even print "premium" on their boarding passes for me (does this really make a difference ?) and give me priority seating when my number is entered into the system - this even extends to blocking out the seat next to me when in economy.

I really think it is BA which is the "thorn" (for want of a better word) in the alliance. Other than lounge access and business class check-in, they rarely, if ever provide any of the other published one-world benefits such as priority seating, priority boarding, etc. I've never had this with any of the other OneWorld airlines I've flown (only AA, CX & QF).