I too was looking for where opatch was located in oracle, and discovered it under:
%ORACLE_HOME%/OPatch/opatch.bat in Windows
Basically, anything under look under $ORACLE_HOME/OPatch - although I would also suggest checking out the README documentation that comes with the patch.
Still stuck troubleshooting on where is opatch located in oracle?
Make sure you're setting the PATH in unix. Do this before applying the patch. How does that work now?
If you need further assistance finding opatch in oracle, I recommend the following site: http://docs.oracle.com/cd/B28359_01/em.111/b31207/oui7_opatch.htm#CEGCJGJD
It goes into a lot more detail about environment variables, prerequisites and requirements.