PL/SQL developer with oracle 32-bit/64-bit client

You'll need to install the two clients into separate Oracle Home locations, for example I've gone for C:\OracleHome and C:\OracleHome32

Then set up an Environment Variable, called TNS_ADMIN with the folder that contains your default TNSnames.ora file as the value (for me it is C:\OracleHome\network\admin)

Keep your preferences in PL/SQL Developer, and make sure you also specify the OCI library (mine is C:\OracleHome32\oci.dll)

Finally, using regedit.exe, add a second key under ORACLE (HKEY_LOCAL_MACHHINE\SOFTWARE\ORACLE). I've called mine KEY_OraClient11g_home1 and KEY_OraClient11g_home2. Create the same 4 strings in the second key, with the appropriate changes to the data (e.g. ORACLE_HOME should have C:\OracleHome32 as it's data field in my example)

Restarting all applications should now let you use PL/SQL Developer seamlessly, whilst also defaulting to the 64-bit Oracle home for your weblogic application.

To fix this, download the 32-bit version of Oracle Instant Client, extract it to a directory such as C:\instantclient.

Next, configure PL/SQL Developer to use this version by clicking on Tools -> Preferences. Under Connection -> Oracle Home, point to the location where you had extracted Instant Client (C:\instantclient), and under Connection -> OCI library, point to the oci.dll file in the same directory (C:\instantclient\oci.dll).

Restart PL/SQL Developer and you should be able to connect.

This is an updated answer specifically for the Oracle 19 instant client and PLSQL Developer 13, which is 64 bit.

To make PL/SQL Dev work with the client, I went to : Configure -> Preferences -> Oracle\connection -> set oracle home to the new home dir, in my case, c:\oracle\product\19.x.

Do the same for the OCI : C:\oracle\product\19.x\instantclient_19_6\oci.dll

If you're migrating from an order version, you probably have built up a list of databases in your tns_names.ora. That will have to be moved over to the new client directory tree.

Manually make a subdir network\admin under the 19.x root. Once restarted, the Database list under 'Define Connection' had my list.