I had to patch Oracle 12c to latest patch set today and it took couple of attempts to me it work.
In our setup, Oracle is running on Windows server.
1) Downloaded patch 26161726 from MetaLink.
2) Go to Windows Services and stop all Oracle services
3) first attempt on "opatch apply" resulted in following error
Following files are active :
D:\app\oracle\product\121~1.0\dbhome_1\bin\oci.dll
A quick search in ProcessExplorer.exe for oci.dll, resulted in 4 processess.
I had to stop "VMware Tools" and "COM+ System Application" services to kill above processes.
Once the OCI.dll reference was cleared, opatch finished successfully.