We use DFP, but yesterday we got a third-party creative from an advertiser that was using Open X server. The ad tags looked like this:
It took me forever to figure out how to set up click tracking with the thing, and I finally discovered it was the variable 'r'. That didn't seem very intuitive, so I thought I'd share. So, if you happen to be using any random ad server and are trying to track clicks with third-party OpenX ad tags, here's how you do it:
<script type="text/javascript">
if (!window.OX_ads) { OX_ads = []; }
OX_ads.push({ "auid" : "12345", "r" : "%%CLICK_URL_UNESC%%" });
</script>
<script type="text/javascript">
document.write('<scr'+'ipt src="http://ox-d.ad-server.com/w/1.0/jstag"><\/scr'+'ipt>');
</script>