There is a complete comparison of Telerik OpenAccess ORM to Entity Framework 4 on the Telerik website:
http://bit.ly/OAvsEF4
Clearly, you're likely to treat that information as biased, so you can find other non-Telerik opinions on other StackOverflow threads:
http://stackoverflow.com/questions/1192234/is-telerik-openaccess-orm-worth-learning
http://stackoverflow.com/questions/440853/compare-and-contrast-nhibernate-and-openaccess-from-telerik
In general, OpenAccess is more mature solution than EF (OA's roots, like NHibernate, trace back to Java) and with 3 major releases per year, it's constantly adding more features and database support. Today, OA supports a litany of popular databases, including SQL Server, Oracle, MySql, and SQLite, and it offers innovative features like the new Round Trip Mapping (forward or reverse map at any time).
Also, don't miss the fact that there is a FREE version of OpenAccess that can be used without restrictions on any free database (like SQL Express):
http://www.telerik.com/community/free-products.aspx