  We investigate the [OII] emission-line as a star formation rate (SFR)
indicator using integrated spectra of 97 galaxies from the Nearby Field
Galaxies Survey (NFGS). The sample includes all Hubble types and contains SFRs
ranging from 0.01 to 100 Msun/yr. We compare the Kennicutt [OII] and H-alpha
SFR calibrations and show that there are two significant effects which produce
disagreement between SFR[OII] and SFR(H-alpha): reddening and metallicity.
Differences in the ionization state of the ISM do not contribute significantly
to the observed difference between SFR([OII]) and SFR(H-alpha) for the NFGS
galaxies with metallicities log(O/H)12>8.5. The Kennicutt [OII]-SFR relation
assumes a typical reddening for nearby galaxies; in practice, the reddening
differs significantly from sample to sample. We derive a new SFR([OII])
calibration which does not contain a reddening assumption. Our new SFR([OII])
calibration also provides an optional correction for metallicity. Our SFRs
derived from [OII] agree with those derived from H-alpha to within 0.03-0.05
dex. We apply our SFR([OII]) calibration with metallicity correction to two
samples: high-redshift 0.8<z<1.6 galaxies from the NICMOS H-alpha survey, and
0.5<z<1.1 galaxies from the Canada-France Redshift Survey. The SFR([OII]) and
SFR(H-alpha) for these samples agree to within the scatter observed for the
NFGS sample, indicating that our SFR([OII]) relation can be applied to both
local and high-z galaxies. Finally, we apply our SFR([OII]) to estimates of the
cosmic star formation history. After reddening and metallicity corrections, the
star formation rate densities derived from [OII] and H-alpha agree to within
30%.
