  We present the results of our abundance analysis of Fe, Ni, and O in high
S/N, high-resolution VLT/UVES and McDonald/2dcoude spectra of nine dwarfs and
three giants in the Hyades open cluster. The difference in Fe abundances
derived from Fe II and Fe I lines ([Fe II/H] - [Fe I/H]) and Ni I abundances
derived from moderately high-excitation (Chi ~ 4.20 eV) lines are found to
increase with decreasing T_eff for the dwarfs. Both of these findings are in
concordance with previous results of over-excitation/ionization in cool young
dwarfs. Oxygen abundances are derived from the Lambda 6300 [O I] line, with
careful attention given to the Ni I blend. The dwarf O abundances are in
star-to-star agreement within uncertainties, but the abundances of the three
coolest dwarfs (4573 < T_eff < 4834 K) evince an increase with decreasing
T_eff. Possible causes for the apparent trend are considered, including the
effects of over-dissociation of O-containing molecules. O abundances are
derived from the near-UV Lambda 3167 OH line in high-quality Keck/HIRES
spectra, and no such effects are found- indeed, the OH-based abundances show an
increase with decreasing T_eff, leaving the nature and reality of the cool
dwarf [O I]-based O trend uncertain. The mean relative O abundance of the six
warmest dwarfs (5075 < T_eff < 5978 K) is [O/H] = +0.14 +/- 0.02, and we find a
mean abundance of [O/H] = +0.08 +/- 0.02 for the giants. Thus, our updated
analysis of the Lambda 6300 [O I] line does not confirm the Hyades giant-dwarf
oxygen discrepancy initially reported by King & Hiltgen (1996), suggesting the
discrepancy was a consequence of analysis-related systematic errors....
