  Realistic models based on the renormalizable grand unified theories have
varieties of scalars, many of which are capable of mediating baryon ($B$) and
lepton ($L$) number non-conserving processes. We identify all such scalar
fields residing in ${\bf 10}$, $\overline{\bf 126}$ and ${\bf 120}$ dimensional
irreps of $SO(10)$ which can induce baryon and lepton number violating
interactions through the leading order $d=6$ and $d=7$ operators. Explicitly
computing their couplings with the standard model fermions, we derive the
effective operators including the possibility of mixing between the scalars
stemming from a given representation. We find that such interactions at $d=6$
are mediated by only three sets of scalars: $T(3,1,-1/3)$, ${\cal T}
(3,1,-4/3)$ and $\mathbb{T}(3,3,-1/3)$ and their conjugates. In the models with
${\bf 10}$ and $\overline{\bf 126}$, only the first has appropriate couplings
to mediate the proton decay. While ${\cal T}$ and $\mathbb{T}$ can induce
baryon number violating interactions when ${\bf 120}$ is present, ${\cal T}$
does not contribute to the proton decay at tree level because of its flavour
antisymmetric coupling. Three additional colour triplets and their conjugates
can mediate nucleon decay via $d=7$ operators which violate also the $B-L$. We
give general expressions for partial widths of proton in terms of the
fundamental Yukawa couplings and use these results to explicitly compute the
proton lifetime and branching ratios for the minimal non-supersymmetric
$SO(10)$ model based on ${\bf 10}$ and $\overline{\bf 126}$ Higgs. We find that
the proton preferably decays into $\overline{\nu}\, K^+$ or $\mu^+\, K^0$ and
list several distinct features of scalar mediated proton decay. If the latter
dominates over the gauge mediated contributions, the proton decay spectrum
provides a direct probe to the flavour structure of the underlying grand
unified theory.
