You can earn BA tier points by flying other oneworld airlines. Some booking classes are sometimes excluded from earning but most classes do earn.

You can find out which classes are eligible here:

https://www.britishairways.com/en-gb...-avios/flights

You can calculate the number of Avios (redeemable miles) and Tier Points (for status) that you'll earn from a particular flight here:

https://www.britishairways.com/trave...r/public/en_gb