  We study three dimensional N=2 supersymmetric QCD theories with O(N_c) gauge
groups and with N_f chiral multiplets in the vector representation. We argue
that for N_f < N_c-2 there is a runaway potential on the moduli space and no
vacuum. For N_f >= N_c-2 there is a moduli space also in the quantum theory,
and for N_f >= N_c-1 there is a superconformal fixed point at the origin of
this moduli space that has a dual description as the low-energy fixed point of
an O(N_f-N_c+2) gauge theory. We test this duality in various ways; in some
cases the duality for an O(2) gauge theory may be related to the known duality
for U(1) gauge theories. We also discuss real mass deformations, which allow to
connect theories with a different Chern-Simons level. This allows us to connect
our duality with the known duality in O(N_c) theories with a Chern-Simons term
of level k, where the dual gauge group is given by O(N_f+|k|-N_c+2).
