How can I have common login for all my web apps under the same domain?

I have a website in which I have different web applications like opencart, osclass, wordpress and vanilla forums. How can I make same user login across all my web apps. I want to register as user and as bussiness account across this web applications. You can check [my website][1].

I used OAuth login with facebook and google. But still the user login is not that much satisfactory.