OB=(n, 1), OC=(5,-1), then CB = ob-oc = (n-5,2), and similarly CA=(-7, m+ 1).

Because of the * * line of A, B and C, (n-5)/2=(-7)/(m+ 1).

With OA vertical OB, OA*OB=0, that is, -2n+m=0,

Simultaneous equations can give m = 6 and n = 3；; Or m = 3 and n = 3/2.

So the value of m+n is 9 or 9/2.