We provided standard login/registration forms for providing email and password, but I'd also like to allow user use my website after oAuth authentication.
Should they be able to register or login via oAuth?
Where's the right place to inform user about possibility of use FB, G+ or LinkedIn. In login or register form?
Using oAuth for logging in with social media is normally used in two separate ways.
An option to create an account with an existing social media account so user normally don't have to go through the tedious process of filling out forms and authenticating email addresses.
An option to link their [your-web-application-name]account with one or several social media accounts. That way, when they enter the website and they are already logged in to, let's say Facebook, they're automatically log in to your web application.
In case of case 1, I would suggest only inform the user at registration since it's of no use to existing members.
In case of case 2, I would suggest both. Display it beneath the log in form and at the end of a registration form.
