This happened to me recently when testing a 188.8.131.52 upgrade
As I know for sure this machine's inventory is good I decided to investigate.
Metalink Note 1292347.1 is a useful reference
It turns out the inventory oraInst.loc file used to be able to get away with only inst_loc line , now you need inst_group as well.
on Linux this is usually oinstall.
So oraInst.loc should look like
Once that is done re-run the installer and it should now start up OK.