Ok. I've narrowed it down to the html_header.php file included with cjloader. I uploaded the original cjloader file again (over my altered one) and the One Page Checkout worked fine, but at least two of the style sheets failed to load (the most important two!), so my site doesn't look right.
Stylesheet.css has this at the top, which calls up two extra stylesheets:
These are the two missing files. I guess I could just copy the info from them both into stylesheet.css, but would like to figure out why they aren't loading in case other stylesheets are failing to load too.