What is the volume of a parallelopiped with adjacent vertices at O(0, 0, 0), A(1, 1, -2), B(3, 1, -1) and C(2, -1, 1)?

I got the vectors:

OA= (1,1,-2)

OB= (3,1,-1)

OC= (2,-1,1)

but I am not sure which 2 vectors I must cross (and obviously the other one will get dotted). How do you know which 2 vectors are in the same plane in order to cross them?