We report an optical photometric survey of 0.89 deg ^ { 2 } 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 \sim 160 members in the mass range ( 0.2 \leq M \leq 1.0 M _ { \odot } ) .
The cluster has a radius of \sim 3-5 pc and an estimated age of 2.5 \pm 0.3 Myrs .
We estimate that the total mass of the cluster is 225 \pm 30 M _ { \odot } .
This mass is similar to the estimated mass of the \sim 5 \times 10 ^ { 5 } year old cluster NGC 2024 .
NGC 2024 and \sigma Ori appear to be a well matched pair of clusters , except for the \sim 2 Myr difference in their ages .