Is there a demo version of OFP?

Yes there are both single player and multi-player versions of the demo available! 

To download the demos, go to BIS' OFP demo download page. If you're looking specifically for the OFP demo's original version, read here.

NOTE: the zipped single player demo file is 61MB and the multi-player demo is 50MB. If you don't have a broadband link, be patient till the download is completed.

If you decide to buy the full retail version,  just remember to uninstall the demo(s) first before installing the full version afterwards.