Orks are highly resistant against the influences of Chaos, but in very rare cases they can be "infested".
For my personal opinion you should look in this thread:
viewtopic.php?f=4&t=1129
The german version of the lexicanum contains an article based on the orc background book "freebooterz" which was released during the first edition of Warhammer 40k. It speaks about the "Stormboyz of Khorne". While normal Stormboyz are young Orks who rebell against the ork society and will go back to the normal orkish life sometimes, there are some Orks that will ever be stormboyz. Most of them make their own clans and life a bit outside from the ork society 'cause their military organisation isn't very orkish.
But a few of them start to worship Khorne their militarism can only be fullfilled by collection skulls in the name of the bloodgod. Those ones life outside the ork society and normal orks will definatly kill them, when they can 'cause worshipping the chaos the worst thing you can do as ork (maybe besides being a hippie

).
Here is the article (german only!):
http://wh40k.lexicanum.de/wiki/Stormboyz_des_KhorneAnd here is the official (!) miniature:

Scythe
EDIT: all this is just for 40k but I think you can make a story for fantasy out of this
