There has been several scripts for that (GetVideo, Tubexx...), but since the YouTube defaulted to the HTML5 and made several changes once again, they haven't kept updated lately. I still would like to see GetVideo updated to current YouTube behaviour even though OWB plays them mostly, but well...

But to why your OWB doesn't play them anymore... check if Chrysalis still had Youtube HTML5 Converter script enabled, and disable it if it is, because it's not needed anymore. You only should have the Youtube Fullscreen Fix enabled (and the latest version of it).