  OWL uses the Earth's atmosphere as a vast calorimeter to fully enable the
emerging field of charged-particle astronomy with high-statistics measurements
of ultra-high-energy cosmic rays (UHECR) and a search for sources of UHE
neutrinos and photons. Confirmation of the Greisen-Zatsepin-Kuzmin (GZK)
suppression above ~4 x 10^19 eV suggests that most UHECR originate in
astrophysical objects. Higher energy particles must come from sources within
about 100 Mpc and are deflected by ~1 degree by predicted
intergalactic/galactic magnetic fields. The Pierre Auger Array, Telescope Array
and the future JEM-EUSO ISS mission will open charged-particle astronomy, but
much greater exposure will be required to fully identify and measure the
spectra of individual sources. OWL uses two large telescopes with 3 m optical
apertures and 45 degree FOV in near-equatorial orbits. Simulations of a
five-year OWL mission indicate ~10^6 km^2 sr yr of exposure with full aperture
at ~6 x 10^19 eV. Observations at different altitudes and spacecraft
separations optimize sensitivity to UHECRs and neutrinos. OWL's stereo event
reconstruction is nearly independent of track inclination and very tolerant of
atmospheric conditions. An optional monocular mode gives increased reliability
and can increase the instantaneous aperture. OWL can fully reconstruct
horizontal and upward-moving showers and so has high sensitivity to UHE
neutrinos. New capabilities in inflatable structures optics and silicon
photomultipliers can greatly increase photon sensitivity, reducing the energy
threshold for neutrino detection or increasing viewed area using a higher
orbit. Design trades between the original and optimized OWL missions and the
enhanced science capabilities are described.
