  The Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS)
is a NASA Astrophysics MIDEX-class mission concept, with the stated goal of
following water from galaxies, through protostellar systems, to Earth's oceans.
This paper details the protoplanetary disk science achievable with OASIS.
OASIS's suite of heterodyne receivers allow for simultaneous, high spectral
resolution observations of water emission lines spanning a large range of
physical conditions within protoplanetary disks. These observations will allow
us to map the spatial distribution of water vapor in disks across evolutionary
stages and assess the importance of water, particularly the location of the
midplane water snowline, to planet formation. OASIS will also detect the H2
isotopologue HD in 100+ disks, allowing for the most accurate determination of
total protoplanetary disk gas mass to date. When combined with the
contemporaneous water observations, the HD detection will also allow us to
trace the evolution of water vapor across evolutionary stages. These
observations will enable OASIS to characterize the time development of the
water distribution and the role water plays in the process of planetary system
formation.
