  Assume that $(L,v)$ is a finite Galois extension of a valued field $(K,v)$.
We give an explicit construction of the valuation ring $\mathcal O_L$ of $L$ as
an $\mathcal O_K$-algebra, and an explicit description of the module of
relative K\"ahler differentials $\Omega_{\mathcal O_L|\mathcal O_K}$ when $L|K$
is a Kummer extension of prime degree or an Artin-Schreier extension, in terms
of invariants of the valuation and field extension. The case when this
extension has nontrivial defect was solved in a recent paper by the authors
with Anna Rzepka. The present paper deals with the complementary (defectless)
case. The results are known classically for (rank 1) discrete valuations, but
our systematic approach to non-discrete valuations (even of rank 1) is new. We
also show that the annihilator of $\Omega_{\mathcal O_L|\mathcal O_K}$ can only
be equal to the maximal ideal $\mathcal M_L$ of $\mathcal O_L$ if the extension
is defectless and $\mathcal M_L$ is principal.
  Using our results from the prime degree case, we characterize when
$\Omega_{\mathcal O_L|\mathcal O_K}=0$ holds for an arbitrary finite Galois
extension of valued fields. As an application of these results, we give a
simple proof of a theorem of Gabber and Ramero, which characterizes when a
valued field is deeply ramified. We further give a simple characterization of
deeply ramified fields with residue fields of characteristic $p>0$ in terms of
the K\"ahler differentials of Galois extensions of degree $p$.
