I've never heard of this problem, so I'm not sure what the easiest solution is, but maybe one of the following will fix it:
Remove, Reinstall Oracle client
Steps here: Cannot load OCI: C:\Oracle\product\12.2.0\client_1\BIN\OCI.dll
Reset Toad's user files
- From Toad's main menu: Utilities -> Copy User Settings
- Choose the option to Reset to a clean set
- Click OK
- Choose Yes and then OK on the next two dialogs
This will restore Toad to a clean state as if it were freshly installed. Your existing user files will be backed up to %APPDATA%\Quest Software\Toad for Oracle\13.1_bak.
Now, start Toad, make connection, and try again. If it works you can close Toad and copy any configuration files over from the backed up set to the new ( %APPDATA%\Quest Software\Toad for Oracle\13.1 ) . I'd recommend copying only what is absolutely necessary. For example...
- CONNECTIONS.* (all of your saved connections)
- SavedSQL.dat (SQL Recall)
- ToadActions.dat (Automation Designer apps and actions)
- Workspaces.dat (saved Workspaces)
- *.tpr (Project Manager projects)
If there are some other setting you'd like to find, let me know and I'll add them to the list.
If that doesn't help, maybe reinstall Toad? Usually that doesn't solve anything, but I'm running out of ideas.