Unlike on other platforms, do not set ORACLE_HOME as a fixed environment variable when running Oracle Database on Windows operating systems. This is because the Oracle software determines where executable files reside at run time.

When you invoke an Oracle executable program on Windows, for example sqlplus.exe, the ORACLE_HOME, ORACLE_BASE, and ORACLE_SID variables are determined by the PATH environment variable and the location of the executable program (which Oracle home it resides in). To use SQL*Plus to manage a different database or Oracle ASM instance, click the Windows Start button, select the correct Oracle Home for the instance you want to manage, and then select the SQL*Plus utility.

You can use Oracle Universal Installer (OUI) to specify an Oracle home as the default Oracle home and update the PATH environment variable to point to that Oracle home.