If the OCI file is not configured for
PL/SQL Developer, then configure it.
If the configuration is okay, but
PL/SQL Developer cannot be loaded (usually encountered in a new environment), it is not that the oci.dll file is faulty, but the dynamic library included in the
Microsoft Visual C++ Redistributable Packages is missing.
To check, download the
SQL*PLUS toolkit together with
Oracle Instant Client and run
sqlplus.exe. If it fails, the MSVCR120.dll not found error pops up. Install the corresponding version of
Microsoft Visual C++ Redistributable Packages.