We present results of a large scale , multi-epoch optical survey of the Ori OB1 association , carried out with the QuEST camera at the Venezuela National Astronomical Observatory .
We identify for the first time the widely spread low-mass , young population in the Orion OB1a and OB1b sub-associations .
Candidate members were picked up by their variability in the V -band and position in color-magnitude diagrams .
We obtained spectra to confirm membership .
In a region spanning \sim 68 \Box ^ { \circ } we found 197 new young stars ; of these , 56 are located in the Ori OB1a subassociation and 142 in Ori OB1b .
The spatial distribution of the the low mass young stars is spatially coincident with that of the high mass members , but suggests a much sharper edge to the association .
Comparison with the spatial extent of molecular gas and extinction maps indicates that the subassociation Ori 1b is concentrated within a ring-like structure of radius \sim 2 ^ { \circ } ( \sim 15 pc at 440 pc ) , centered roughly on the star \epsilon Ori in the Orion belt .
The ring is apparent in ^ { 13 } CO and corresponds to a region with an extinction A _ { V } \geq 1 .
The stars exhibiting strong H \alpha emission , an indicator of active accretion , are found along this ring , while the center is populated with weak H \alpha emitting stars .
In contrast , Ori OB1a is located in a region devoid of gas and dust .
We identify a grouping of stars within a \sim 3 \ > deg ^ { 2 } area located in 1a , roughly clustered around the B2 star 25 Ori .
The Herbig Ae/Be star V346 Ori is also associated with this grouping , which could be an older analog of \sigma Ori .
Using using several sets of evolutionary tracks we find an age of 7 - 10 Myr for Ori 1a and of \sim 4 - 6 Myr for Ori OB1b , consistent with previous estimates from OB stars .
Indicators such as the equivalent width of H \alpha and near-IR excesses show that the number of accreting low-mass stars decreases sharply between Ori 1b and Ori 1a .
These results indicate that while a substantial fraction of accreting disks remain at ages \sim 5 Myr , inner disks are essentially dissipated by 10 Myr .