ah, looks like you’re not using the right base url here, which would explain the 404 you see AND why the second URL only results in users logging into your okta org and not your OIDC app.
Can you try using a url like the below instead, that way the request is being made to the authorization server? Note the /oauth2 in the request path, which is missing from your 1st example
https://mycompany.okta.com/oauth2/v1/authorize?client_id=redacted&response_type=code&response_mode=query&redirect_uri=http%3A%2F%2F10.14.80.123%2Fmyapp%2Frestapi%2Fv1%2Fauth%2Fokta%2Fcallback&state=redacted&nonce=redacted