One way is by similar triangles.
We suppose that A,B,C,O are distinct points (otherwise the question isn't very well defined).
In particular, z \ne 0, 1. Then,
\frac{OC}{OA} = \frac{|1|}{|z|},
\frac{BC}{CA} = \frac{|1-1/z|}{|z-1|} =\frac{|1/z||z-1|}{|z-1|},
\frac{OB}{OC} =...