I faced this issue with facebook login on Question2Answer version 1.7.2 and SnowFlat theme.
I identified the issue to exist only when I use Facebook Page Plugin(which is new option for Facebook Likebox). Looks like the scripts for Facebook Page Plugin and Login are interfering when using SnowFlat theme. If another theme/template is activated, the issue does not exist(checked with Snow template).
I placed the second part under "Custom HTML in sidebar box of everypage" (admin>>layout).
After the above change, the Oauth window opens normally and after entering username and password, the pop up closes and the user is able to login to the site.