  We obtained medium-resolution spectra of 336 quasar candidates in the COSMOS
HST/Treasury field using the MMT 6.5-meter telescope and the Hectospec
multi-object spectrograph. Candidates were drawn from the Sloan Digital Sky
Survey (SDSS) DR1 catalog using quasar flags set by the SDSS multi-color quasar
target selection algorithm. In this paper we present our discovery spectra from
1.39 square degrees (69.5% of the COSMOS field) and a discussion of the
selection method and yields. We confirmed 95 quasars, including at least 2
BALs; 80 of these are new quasars that do not appear in previous quasar
confirmation follow-up studies. The candidates additionally included 184
compact emission-line galaxies, a third of which are likely Type 2 AGN, and 12
stars. The quasars span a range in magnitude of 18.3<g<22.5 and a range in
redshift of 0.2<z<2.3. Our results are consistent with a lower limit quasar
surface density from SDSS color selection of 102 per square degree down to
g=22.5 over the entire COSMOS field. This work is the first step toward the
eventual goal of setting up a grid of quasar absorption line probes of the 2
square degree field, and of conducting a complete census of supermassive black
holes in this well-studied survey region. The total quasar count at the
conclusion of this study is 139, making COSMOS one of the most densely-sampled
regions of sky where a grid of quasar sightlines can be used to probe the
intervening volume.
