From their policy at the bottom of the page:
https://www.oneworld.com/airport-lounges
Members of oneworld airline frequent flyer programmes with the equivalent of oneworld Emerald or Sapphire tier status can use lounges offered by oneworld airlines when departing on any flight marketed and operated by any oneworld member airline, regardless of cabin class being flown (exceptions are noted below).
I think often the staff at those lounges might not realise the flight was booked through non-OW carrier (i.e. not marketed by OW member) so still allow access if the system accepts it, but if they are being strict, then they could deny access based on their policy.