There is an official OAuth package for ZF2 already. You can find some information on the ZF2 Packages website.
If you use composer you can install the package by adding the following following the directions on setting up composer here.
For the require packages, just put:
"require": {
"zendframework/zendoauth": "2.0.*"
},
and then run:
php composer.phar install
and it will install the oauth package.
There is also the ReverseOAuth2 library which looks good. I found that on the community supported site modules.zendframework.com.
If you want to download directly, the stable version at the time of writing can be found at: https://packages.zendframework.com/composer/zendframework-zendoauth-2.0.2-release-2.0.2-91b73f.zip
Hope that helps.