  Multimessenger observations of the binary neutron star merger GW170817 have
enabled the discovery of a diverse array of electromagnetic counterparts to
compact binary mergers, including an unambiguous kilonova, a short gamma-ray
burst, and a late-time radio jet. Beyond these counterparts, compact binary
mergers are additionally predicted to be accompanied by prompt low-frequency
radio emission. The successful observation of a prompt radio counterpart would
be immensely valuable, but is made difficult by the short delay between the
gravitational-wave and prompt electromagnetic signals as well as the poor
localization of gravitational-wave sources. Here, we present the first search
for prompt radio emission accompanying a gravitational-wave event, targeting
the binary black hole merger GW170104 detected by the Advanced LIGO and Virgo
gravitational-wave observatories during their second (O2) observing run. Using
the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA), we search
a $\sim900\,\mathrm{deg}^2$ region for transient radio emission within
approximately one hour of GW170104, obtaining an upper limit of
$2.5\times10^{41}\,\mathrm{erg}\,\mathrm{s}^{-1}$ on its equivalent isotropic
luminosity between 27-84 MHz. We additionally discuss plans to target binary
neutron star mergers in Advanced LIGO and Virgo's upcoming O3 observing run.
