From the OneWorld website:
The guest must also be travelling on a flight operated and marketed by a oneworld airline. Access is available on the day of travel (or before 06:00am the following day), at the airport from which the oneworld flight on which the eligible customer is travelling departs.
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).
(Bolding mine)
https://www.oneworld.com/airport-lou...-access-policy
The issue on your trip is that you were not departing on a OneWorld operated and marketed flight, and you were not connecting between two OneWorld marketed and operated flights. It was correct of them to deny you access.