My reason for not implementing my own login is it may not be as secure as the biggies in the business, i don't have to deal with the mess of activation/reset password thing, OAuth is fairly simple and less complicated to implement.
I signup to other sites using OAuth just to avoid remembering another user/pass, but do you think its a good idea to completely rely on Oauth?
I can add the help page link / button to assure user why i am using OAuth but still will it affect the user signups and BTW all services will be free on my site.