This can be broadly split into 2 categories where are OER stored and how can I identify OER in a web search.
There are a number of repositories out there that only store open content so a search of that site will always return openly licensed content. You can find a list of some of the repositories here.
There are also lots of non site specific search engines out there and some are integrating a search facility that provides the capability to search on licence. The biggest of these is Google, there are other search engines like Xpert, from Nottingham University, that aggregate a search across a number of open databases. In addition you have search engines in non exclusively open repositories like YouTuube and Flickr that allow you to search on licence.