[Answered ]-"self.fields['field_name'].queryset" in Djano, can it be applied to a OneToOne field?

You can access the related Profile of a User with:

myuser.profile

indeed, a OneToOneField is in essence a ForeignKey with a unique=True constraint. This means that for a User there is at most one related Profile. The related_name=… parameter [Django-doc] by default is the name of the class (here Profile) in lowercase (so profile). You can thus access the related Profile with .profile. It will raise an AttributeError in case no such Profile exists.

The modeling of manager however has redundant data: you do not need to specify both the User and the Profile, you can determine the other by the first. By introducing duplicate data, you make it possible that in the future Managers exist where the user is updated, but the user_profile is not and vice versa. If you need the profile and you store the user, you can access it with mymanager.user.profile, and if you store the user_profile, you can access it with mymanager.user_profile.user. Storing both thus makes not much sense.

Note: It is normally better to make use of the settings.AUTH_USER_MODEL [Django-doc] to refer to the user model, than to use the User model [Django-doc] directly. For more information you can see the referencing the User model section of the documentation.