You can use the fromUri query parameter to redirect the user after both activation and password reset. You will need to:
- Update the email template which is sent to the user to include the fromUri parameter in the activation link so it would look like: ${activationLink}?fromURI=https://myoidcapp.com
- Add the redirect uri to your trusted origins in OKTA under security -> API -> Trusted Origin -> Add Origin
This reply has more information: Redirect to Login or Application After Activation
Hope this helps.