The oneworld website does say the following:

“Connecting between oneworld marketed and operated flights: First and Business Class customers connecting on the same day of travel, or before 06:00am the following day, can access the lounge when travelling between an international long haul (a oneworld international long haul flight is defined as an international flight marketed and operated by any oneworld carrier with a scheduled flight time longer than 5 hours) and an international short haul or domestic flight (and vice-versa).Lounge access will be determined on the international long haul ticketed flight (either First of Business Class) regardless of the ticketed class of travel on the international short haul or domestic flight. You must be prepared to show your boarding pass or itinerary showing travel in First or Business class on the international long haul flight, in order to access the lounge before your international short haul or domestic flight.”

You might have luck talking your way in using this wording, since it doesn’t state anywhere that you must be on the same ticket. I’ve gotten into AA lounges many times with separate tickets. In Spain I’ve found that you can often get your way if you’re friendly but persistent, but it only if you speak fluent Spanish.