Well, it's your fault if your oxen are taken from under your nose by doing this. Considering the amount of bandit PCs for NCE, let's see what happens to people who actually get caught doing this. >=D
I am not sure you can steal the ox as i think you will still getthis ox is allready claimed by someone message. But you can allways kill and loot the ox
If it's in your party, then I'm reasonably sure someone can't steal it from you. Killing it and looting its corpse is viable though... mind you, unless you have your own ox or herculean strength, you're not likely to be able to carry the stuff and make a quick getaway.