  We report the serendipitous discovery of an [O III] $\lambda\lambda$4959/5007
and H$\alpha$ line emitter in the Epoch of Reionization (EoR) with the JWST
commissioning data taken in the NIRCam wide field slitless spectroscopy (WFSS)
mode. Located $\sim$55" away from the flux calibrator P330-E, this galaxy
exhibits bright [O III] $\lambda\lambda$4959/5007 and H$\alpha$ lines detected
at 3.7, 9.9 and 5.7$\sigma$, respectively, with a spectroscopic redshift of
$z=6.112\pm0.001$. The total H$\beta$+[O III] equivalent width is 664$\pm$98
\r{A} (454$\pm$78 \r{A} from the [O III] $\lambda$5007 line). This provides
direct spectroscopic evidence for the presence of strong rest-frame optical
lines (H$\beta$+[O III] and H$\alpha$) in EoR galaxies as inferred previously
from the analyses of Spitzer/IRAC spectral energy distributions. Two spatial
and velocity components are identified in this source, possibly indicating that
this system is undergoing a major merger, which might have triggered the
ongoing starburst with strong nebular emission lines over a timescale of
$\sim$2 Myr as our SED modeling suggests. The tentative detection of He II
$\lambda$4686 line ($1.9\sigma$), if real, may indicate the existence of very
young and metal-poor star-forming regions with a hard UV radiation field.
Finally, this discovery demonstrates the power and readiness of the JWST/NIRCam
WFSS mode, and marks the beginning of a new era for extragalactic astronomy, in
which EoR galaxies can be routinely discovered via blind slitless spectroscopy
through the detection of rest-frame optical emission lines.
