  We report an optical photometric survey of 0.89 sq. degrees of the Orion OB1b
association centered on sigma Ori. This region includes most of the sigma Ori
cluster, the highest density region within Orion OB1b. We have developed a
statistical procedure to identify the young, low-mass pre-main sequence
population of the association. We estimate that the cluster has ~160 members in
the mass range (0.2<M<1Msun). The cluster has a radius of ~3.5 pc and an
estimated age of 2.5+/-0.3 Myr. We estimate that the total mass of the cluster
is 225+/-30 Msun. This mass is similar to the estimated mass of the ~0.5 Myr
old cluster NGC 2024. NGC 2024 and sigma Ori appear to be a well matched pair
of clusters, except for the ~2 Myr difference in their ages.
