From Rap Dictionary

>===noun=== ox

A razor blade, used as a knife, usually in jail. A radio is a box, a razor blade is an ox -- Big L (Ebonics) [1] The term 'OX' originated in american prisons. Presumably it is a metaphor for the strength represented by the razorblade. A Boxcutter can also be referred to as an 'OX' since it holds a razorblade. Possibly, there's a link between the word 'OX' and the word Boxcutter. Who can say for sure?

---