For this example, we will use the Oracle Foreign Data Wrapper, oracle_fdw, whose version is 2.0.0.
You must have obtained and installed the required Oracle software as specified in the oracle_fdw documentation at https://github.com/laurenz/oracle_fdw/blob/ORACLE_FDW_2_0_0/README.oracle_fdw#L497.
The oracle_fdw wrapper is available in the PostgreSQL Extension Network, so you can follow the straightforward installation procedure described in the Installing modules from PGXN section of the Adding an external module to PostgreSQL recipe in Chapter 3, Configuration.
Obviously, you must have access to an Oracle database server.