The CSS stylesheet thats empty solved the solution. So presumably this is css related, and it sounds like something thats cached. So I looked in /tmp, but theres no /tmp/501 anymore.
Running lsof on the OW process showed these cachefiles actively opened:
$ lsof -p 70218|grep Cache
OmniWeb 70218 kai txt REG 14,2 13556568 650112 /private/var/folders/uh/uh90jpC2Fla79FXqyFLE-++++TI/-Caches-/com.apple.ATS/annex_aux
OmniWeb 70218 kai txt REG 14,2 19336 650095 /private/var/folders/uh/uh90jpC2Fla79FXqyFLE-++++TI/-Caches-/com.apple.IntlDataCache.le.sbdl
OmniWeb 70218 kai txt REG 14,2 97968 3019769 /System/Library/Caches/com.apple.IntlDataCache.le.kbdx
OmniWeb 70218 kai txt REG 14,2 1355776 3178969 /Library/Caches/com.apple.LaunchServices-023501.csstore
OmniWeb 70218 kai 6u REG 14,2 5276672 2634109 /Users/kai/Library/Caches/com.omnigroup.OmniWeb5/WebKitCache/Cache.db
Mkay, so there is a file that has 'css' in it... would that be it?