Both should work with WordOps.
Having re-read your initial post, it looks like you have to add
/var/cache/nginx to the two PHP pool files WordOps uses - by default the path is set to
/var/run/nginx-cache. The two files are located here:
Add it to the end of this line like so:
php_admin_value[open_basedir] = "/var/www/:/usr/share/php/:/tmp/:/var/run/nginx-cache/:/dev/urandom:/dev/shm:/var/lib/php/sessions/:/var/cache/nginx/"