I looked at the Github Om page but it is not clear which version I should be using?
see https://github.com/swannodette/om#using-it
Your project.clj should include something like the following:
(defproject foo "0.1.0"
...
:dependencies [[org.clojure/clojure "1.5.1"]
[org.clojure/clojurescript "0.0-2173"]
[om "0.5.3"]]
...)