  (Abridged) We present deep, narrow-band images of the redshift 0.41 galaxy
cluster Abell 851 (Cl 0939+4713) which isolate [OII] 3726,29 line emission. The
[OII]-selected galaxies lie within a projected clustercentric distance of 2
Mpc. The sample is complete to magnitude m_{5129}(AB) = 24.0$ for galaxies with
emission-line equivalent widths greater than about 11\angs in the observed
frame. Color, brightness, and equivalent-width criteria are used to identify
foreground interlopers (steep 4000 A break across the filters) and candidates
for Ly-alpha emission at z=3.31. The number density of star-forming galaxies
toward Cl~0939+4713 is three to four times higher than that of a similar column
through the field at z=0.4. Galaxies in the cluster sample have lower [OII]
equivalent widths than similarly selected field galaxies at $z \sim 0.4$
however, and we suggest that while the density of galaxies is higher in the
cluster the star-formation activity in individual galaxies is supressed
relative to their counterparts in the field. The relatively small enhancement
in the surface density of the [OII]-selected population toward the cluster core
leads us to believe the [OII]-selected galaxies reside in the outer regions of
the cluster. We identify a population of starburst galaxies among the
[OII]-emitters based on their blue g-i color. The majority of the
[OII]-selected galaxies appear to be related to the normal spiral galaxy
population, but the bursting population is dominated by intrinsically faint
galaxies of below average size. We suggest that this population is composed
primarily of dwarf galaxies and discuss their relation to the dwarf galaxies in
present day clusters.
