are there any ways to reverse engineer Oracle trigger or sort of decompile it?
This may be done querying user_triggers or dba_triggers as such:
SELECT trigger_body FROM user_triggers where trigger_name = 'THENAME';
or
SELECT DBMS_METADATA.GET_DDL('TRIGGER','....') FROM DUAL
If the trigger code calls other code that is Wrapped (obsufacted) you will need to unwrap it, see this article or do a google search for "oracle unwrap pl/sql"
You can also use a Oracle managment or development tool to get the trigger source code.
Sql*plus should be able to solve that. You can query the system tables to get the body text.