Hello,
If you are travelling to international destinations you almost always you need to have an onward or returnticket.
Doesent metter if you flying out of the country on another airline.
I flow once on a oneway ticket from lima to san salvador and they asked me to present another ticket out of the country.
I had an international bus ticket to Guatemala which was not accepted.
Then i got angry and presented my 5 other tickets out of mexico then to the usa and canada and europe and then i finnaly i got my boarding pass.