The file /opt/postgresql/9.6/lib/postgresql/oracle_fdw.so is available.
I have also added "include /usr/lib/oracle/12.1/client64/lib" in /etc/ld.so.conf
I still see below error when I perform "create extension oracle_fdw;"
ERROR: could not load library "/opt/postgresql/9.6/lib/postgresql/oracle_fdw.so": libclntsh.so.12.1: cannot open shared object file: No such file or directory