We measure the evolution of the [ O ii ] \lambda 3727 luminosity function at 0.75 < z < 1.45 using high-resolution spectroscopy of \sim 14 , 000 galaxies observed by the DEEP2 galaxy redshift survey . We find that brighter than L _ { \mathrm { [ OII ] } } = 10 ^ { 42 } ~ { } \mathrm { erg~ { } s ^ { -1 } } the luminosity function is well-represented by a power law dN / dL \propto L ^ { \alpha } with slope \alpha \sim - 3 . The number density of [ O ii ] -emitting galaxies above this luminosity declines by a factor of \gtrsim 2.5 between z \sim 1.35 and z \sim 0.84 . In the limit of no number-density evolution , the characteristic [ O ii ] luminosity , L _ { \mathrm { [ OII ] } } ^ { * } , defined as the luminosity where the space density equals 10 ^ { -3.5 } ~ { } \mathrm { dex ^ { -1 } ~ { } Mpc ^ { -3 } } , declines by a factor of \sim 1.8 over the same redshift interval . Assuming that L _ { \mathrm { [ OII ] } } is proportional to the star-formation rate ( SFR ) , and negligible change in the typical dust attenuation in galaxies at fixed [ O ii ] luminosity , the measured decline in L _ { \mathrm { [ OII ] } } ^ { * } implies a \sim 25 \% per Gyr decrease in the amount of star formation in galaxies during this epoch . Adopting a faint-end power-law slope of -1.3 \pm 0.2 , we derive the comoving SFR density in four redshift bins centered around z \sim 1 by integrating the observed [ O ii ] luminosity function using a local , empirical calibration between L _ { \mathrm { [ OII ] } } and SFR , which statistically accounts for variations in dust attenuation and metallicity among galaxies . We find that our estimate of the SFR density at z \sim 1 is consistent with previous measurements based on a variety of independent SFR indicators .