how to find which user performed DML operations in a table in oracle?
The answer is auditing.

Monitoring Database Activity with Auditing

Traditional auditing:

audit all on owner.mytable by access;


Unified auditing:

create audit policy mypolicy actions all on owner.mytable;
audit policy mypolicy;


If audit_trail is set to DB or DB,EXTENDED, results can be queried from DBA_AUDIT_TRAIL in case of traditional auditing.
With Unified auditing, results can be queried from UNIFIED_AUDIT_TRAIL.