  We present a detailed report on sterile neutrino oscillation and U-235
antineutrino energy spectrum measurement results from the PROSPECT experiment
at the highly enriched High Flux Isotope Reactor (HFIR) at Oak Ridge National
Laboratory. In 96 calendar days of data taken at an average baseline distance
of 7.9 m from the center of the 85 MW HFIR core, the PROSPECT detector has
observed more than 50,000 interactions of antineutrinos produced in beta decays
of U-235 fission products. New limits on the oscillation of antineutrinos to
light sterile neutrinos have been set by comparing the detected energy spectra
of ten reactor-detector baselines between 6.7 and 9.2 meters. Measured
differences in energy spectra between baselines show no statistically
significant indication of antineutrinos to sterile neutrino oscillation and
disfavor the Reactor Antineutrino Anomaly best-fit point at the 2.5$\sigma$
confidence level. The reported U-235 antineutrino energy spectrum measurement
shows excellent agreement with energy spectrum models generated via conversion
of the measured U-235 beta spectrum, with a $\chi^2$/DOF of 31/31. PROSPECT is
able to disfavor at 2.4$\sigma$ confidence level the hypothesis that U-235
antineutrinos are solely responsible for spectrum discrepancies between model
and data obtained at commercial reactor cores. A data-model deviation in
PROSPECT similar to that observed by commercial core experiments is preferred
with respect to no observed deviation, at a 2.2$\sigma$ confidence level.
