  A new class of extragalactic astronomical sources discovered in 2021, named
Odd Radio Circles (ORCs, Norris et al. 2021), are large rings of faint, diffuse
radio continuum emission spanning ~1 arcminute on the sky. Galaxies at the
centers of several ORCs have photometric redshifts of z~0.3-0.6, implying
physical scales of several 100 kiloparsecs in diameter for the radio emission,
the origin of which is unknown. Here we report spectroscopic data on an ORC
including strong [OII] emission tracing ionized gas in the central galaxy of
ORC4 at z=0.4512. The physical extent of the [OII] emission is ~40 kpc in
diameter, larger than expected for a typical early-type galaxy (Pandya et al,
2017) but an order of magnitude smaller than the large-scale radio continuum
emission. We detect a ~200 km/s velocity gradient across the [OII] nebula, as
well as a high velocity dispersion of ~180 km/s. The [OII] equivalent width
(EW, ~50 Ang) is extremely high for a quiescent galaxy. The morphology,
kinematics, and strength of the [OII] emission are consistent with the infall
of shock ionized gas near the galaxy, following a larger-scale, outward moving
shock driven by a galactic wind. Both the extended optical and radio emission,
while observed on very different scales, may therefore result from the same
dramatic event.
