Here is what you need to do to disconnect from Google Oauth:
Sign in to Optimal Workshop using google Oauth.
Generate a new Oauth link by visiting https://www.optimalworkshop.com/a/5rf1ty6s/user/profile and clicking on "Disconnect Google account".
This step above will cause a Oauth disconnect email to be sent to you. Open this recent email and click the disconnect link. (you will need to ensure you're still logged in to Optimal Workshop via google oauth when opening this link). When opening the disconnect link from the email, if you are asked to sign in, do so using your Google account.
Enter your new password.
You should be good to go.