  We present late-time ($\sim$240-260 days after peak brightness) optical
photometry and nebular (+236 and +264 days) spectroscopy of SN 2018oh, the
brightest Type Ia supernova (SN Ia) observed by the Kepler telescope. The
Kepler/K2 30-minute cadence observations started days before explosion and
continued past peak brightness. For several days after explosion, SN 2018oh had
blue "excess" flux in addition to a normal SN rise. The flux excess can be
explained by the interaction between the SN and a Roche-lobe filling
non-degenerate companion star. Such a scenario should also strip material from
the companion star, that would emit once the SN ejecta become optically thin,
imprinting relatively narrow emission features in its nebular spectrum. We
search our nebular spectra for signs of this interaction, including close
examination of wavelengths of hydrogen and helium transitions, finding no
significant narrow emission. We place upper limits on the luminosity of these
features of $2.6,\ 2.9\ \mathrm{and}\ 2.1\times10^{37}\ \mathrm{erg\ s^{-1}}$
for H$\alpha$, He I $\lambda$5875, and He I $\lambda$6678, respectively.
Assuming a simple model for the amount of swept-up material, we estimate upper
mass limits for hydrogen of $5.4\times10^{-4}\ \mathrm{M_{\odot}}$ and helium
of $4.7\times10^{-4}\ \mathrm{M_{\odot}}$. Such stringent limits are unexpected
for the companion-interaction scenario consistent with the early data. No known
model can explain the excess flux, its blue color, and the lack of late-time
narrow emission features.
