Ox, whose real name is Marvin, is a mill hand at Ackerman & Hook. He's one of the first of many male figures who helps Tom, taking the thirteen-year-old boy under his wing when he starts work at the mill and teaching him kindly and patiently how to do the job.
Plus, he teaches Tom how to be a good sport. When the mill hands receive their Christmas bonuses and Tom receives much less than the Moucheauds, Ox gently explains that Tom has to consider that he has no prior mill experience and that he has only been working there a few months: "You've got to take that into account, you see. Right now you're at the bottom of the ladder. A lot of people mightn't have given you anything" (17.22). That makes Tom feel better.
As is the case with many of the other men who help Tom, Ox is positively impacted by the boy. We learn that Ox is kind of a lonely man. His two children are grown up and have moved away, and he spends his time away from the mill taking care of his sickly wife. With Tom, he has someone to talk to and share lunch with.
As the story goes on, Ox pretty much falls out of the plot, though he does come to help with the barn-raising and he also helps Tom find his new wagon. Those two instances are reminders of Ox's early and consistent role in helping Tom.