Here's what I'm picturing:
- User opens mobile app.
 - User clicks login button
 - User enters username and password into mobile app
 - App hits the 
/oauth2/tokenendpoint with user/pass - FA responds with 
twoFactorIdand array of all enabled two factor methods, including theidof those methods - App POSTs to 
/api/two-factor/send/{{twoFactorId}}, including the method ID from the above response in the body - User sees OTP/code input screen & enters what they have received via email/SMS
 - App sends the code to FusionAuth. It seems there isn't a way to do that via oauth endpoints (
/api/two-factor/loginseems like the only way to go). - FusionAuth sends access/refresh tokens w/ user data back to app
 - App user is now logged in
 
We're now switching this over to /api/login instead, and we'll add every user to the application upon registration (which we hadn't been doing). I think that solves our issues. The question remains whether MFA can be used with OAuth login (steps 5 and 8 seem to indicate that it cannot).