  We develop a strong-disorder renormalization group to study quantum phase
transitions with continuous O$(N)$ symmetry order parameters under the
influence of both quenched disorder and dissipation. For Ohmic dissipation, as
realized in Hertz' theory of the itinerant antiferromagnetic transition or in
the superconductor-metal transition in nanowires, we find the transition to be
governed by an exotic infinite-randomness fixed point in the same universality
class as the (dissipationless) random transverse-field Ising model. We
determine the critical behavior and calculate key observables at the transition
and in the associated quantum Griffiths phase. We also briefly discuss the
cases of superohmic and subohmic dissipations.
