  Oxygen abundances have been derived from the near-IR, high-excitation Lambda
7774 O I triplet in high-resolution, high signal-to-noise spectra of 45 Hyades
dwarfs using standard one dimensional, plane-parallel LTE models. Effective
temperatures of the stellar sample range from 4319-6301 K, and the derived
relative O abundances as a function of T_eff evince a trichotomous morphology.
At T_eff > 6100 K, there is evidence of an increase in the O abundances with
increasing T_eff, consistent with non-LTE (NLTE) predictions. At intermediate
T_eff (5450 < T_eff < 6100 K), the O abundances are flat, and star-to-star
values are in good agreement, having a mean value of [O/H] = +0.25 +/- 0.02;
however, systematic errors at the ~0.10 dex level might exist. The O abundances
for stars with T_eff < 5450 K show a striking increase with decreasing T_eff,
in stark contrast to expectations and canonical NLTE calculations. The cool
Hyades triplet results are compared to those recently reported for dwarfs in
the Pleiades cluster and the UMa moving group; qualitative differences between
the trends observed in these stellar aggregates point to a possible age-related
diminution of triplet abundance trends in cool open cluster dwarfs.
Correlations with age-related phenomena, i.e., chromospheric activity and
photospheric spots, faculae, and/or plages, are investigated. No correlation
with Ca II H+K chromospheric activity indicators is observed. Multi-component
LTE ``toy'' models have been constructed in order to simulate photospheric
temperature inhomogeneities that could arise from the presence of starspots,
and we demonstrate that photospheric spots are a plausible source of the
triplet trends among the cool dwarfs.
