There are two ways to define Oracle home for the PL/SQL developer.
One way is to have it in the environment variables, and point it to the installed directory. This works globally, so any other program/software is going to look for ORACLE_HOME will use this one.
Another way is to use the ORACLE_HOME registry value. You would be able to find it under HKEY_LOCAL_MACHINE -> SOFTWARE -> ORACLE -> ORACLE_HOME ; Considering you have installed Oracle on your PC.
Using this registry needs some tweaks. You need to find the params.ini in your PL/SQL developer installed directory and uncomment the oraclehome and set it to ORACLE_HOME.
In the latter approach, PL/SQL developer first looks at the params.ini file for ORACLE_HOME, so you can have set your ORACLE_HOME variable in your environment variables to some other directories and use another one with your PL/SQL developer.
One way is to have it in the environment variables, and point it to the installed directory. This works globally, so any other program/software is going to look for ORACLE_HOME will use this one.
Another way is to use the ORACLE_HOME registry value. You would be able to find it under HKEY_LOCAL_MACHINE -> SOFTWARE -> ORACLE -> ORACLE_HOME ; Considering you have installed Oracle on your PC.
Using this registry needs some tweaks. You need to find the params.ini in your PL/SQL developer installed directory and uncomment the oraclehome and set it to ORACLE_HOME.
In the latter approach, PL/SQL developer first looks at the params.ini file for ORACLE_HOME, so you can have set your ORACLE_HOME variable in your environment variables to some other directories and use another one with your PL/SQL developer.