Some you'll regularly might see Koz use:
Flowers in the blood (or most of the time: Flower-blood, aka elf or pansy);
You’re ugly and your mother dresses you! (while this may be true for most orcs, I doubt it's anything to be proud of!);
Son of snakes! (decide for yourself);
Listen to my laughter! (when mocking someone);
Your head is empty (true in most cases as well, but still
);
Grom-damn! (popular one, I know);
Possum(-spit) (one of Koz's favourites).
Also, some random insults I picked up back in my dwarvish RPing days. Some of them are actually quite good for orcs too. (<...> is where you just insert a random word like orc or coward or whatnot)
Fossil;
Troll-barf;
Gnome's-armpit;
Poetry-reading <...>;
Axe-breaking <...>;
Elf-kissing <...>;
Tool-snatching <...>;
Clanless <...>;
Drooling <...>;
Incompetent <...>.
That should be plenty for you Poetry-reading possums for now! Go do your homework and come back like the rude orc you are!
Disclaimer: Anyone found using these insults against yours truly will be decapitated on sight. ;>