Sometimes Toad gives these errors if you:
- Start Toad
- Make a connection
- Disconnect but leave Toad running
- Make a connection using a different Oracle client.
If that's not it, and you get the error every time:
- If you have more than one Oracle client installed, maybe you need to clean things up a bit by removing unneeded clients and making sure that the registry is correct. Steps to do that are here: Cannot load OCI: C:\Oracle\product\12.2.0\client_1\BIN\OCI.dll - #7 by noob2019
- If you have only one client installed and are just trying to get it to work for the first time, make sure that the path to oci.dll is in your system path (and you have rebooted since you set that)