We study, both theoretically and experimentally, the occurrence of topological defects in polariton superfluids in the optical parametric oscillator (OPO) regime. We explain in terms of local supercurrents the deterministic behavior of both the onset and dynamics of vortex-antivortex pairs generated by perturbing the system with a pulsed probe. Using a generalized Gross-Pitaevskii equation, including photonic disorder, pumping and decay, we elucidate the reason why topological defects form in couples and can be detected by direct visualizations in multishot OPO experiments.