Supernova (SN) 2018oh (ASASSN-18bt) is the first
spectroscopically-confirmed type Ia supernova (SN Ia) observed in the
$Kepler$ field. The $Kepler$ data revealed an excess emission in its
early light curve, allowing to place interesting constraints on its
progenitor system (Dimitriadis et al. 2018, Shappee et al. 2018b). Here,
we present extensive optical, ultraviolet, and near-infrared photometry,
as well as dense sampling of optical spectra, for this object. SN 2018oh
is relatively normal in its photometric evolution, with a rise time of
18.3$\pm$0.3 days and $\Delta$m$_{15}(B)=0.96\pm$0.03 mag, but it seems
to have bluer $B - V$ colors. We construct the "uvoir" bolometric light
curve having peak luminosity as 1.49$\times$10$^{43}$erg s$^{-1}$, from
which we derive a nickel mass as 0.55$\pm$0.04M$_{\odot}$ by fitting
radiation diffusion models powered by centrally located $^{56}$Ni. Note
that the moment when nickel-powered luminosity starts to emerge is +3.85
days after the first light in the Kepler data, suggesting other origins
of the early-time emission, e.g., mixing of $^{56}$Ni to outer layers of
the ejecta or interaction between the ejecta and nearby circumstellar
material or a non-degenerate companion star. The spectral evolution of
SN 2018oh is similar to that of a normal SN Ia, but is characterized by
prominent and persistent carbon absorption features. The C II features
can be detected from the early phases to about 3 weeks after the maximum
light, representing the latest detection of carbon ever recorded in a SN
Ia. This indicates that a considerable amount of unburned carbon exists
in the ejecta of SN 2018oh and may mix into deeper layers.
