ORA - 00933 confusion with inner join and "as"

Just remove the AS keyword

select 
   P.carrier_id, 
   O.order_id, 
   O.aircraft_id, 
   O.quantity
from 
   orderline O
inner join purchaseorder P
   on O.order_id = P.carrier_id;

You are not allowed to insert keyword as between table name and its alias.