For Postgres_Fdw we have below command to increase the fetch size.
ALTER SERVER ServerName OPTIONS ( fetch_size='50000');
Do we have a similar command for oracle_fdw?
The oracle_fdw project isn't even handled in the core PostgreSQL project so none of our lists are really a good location to post this question. But if you do want to have a chance at a response you should post to the -general list where people actually discuss topics of general interest to the community, not the SQL list which is basically a list for beginners who are trying to learn SQL.