We present results of our large scale , optical , multi-epoch photometric survey of \sim 180 square degrees across the Orion OB1 association , complemented with extensive follow up spectroscopy .
Our focus is mapping and characterizing in an uniform way the off-cloud , low-mass , pre-main sequence populations .
We report 2064 , mostly K and M-type , confirmed T Tauri members .
Most ( 59 % ) are located in the OB1a subassociation , 27 % are projected onto the OB1b subassociation , and the remaining 14 % are located within the confines of the A and B molecular clouds .
There is significant structure in the spatial distribution of the young stars .
We characterize two new clusterings of T Tauri stars , the HD 35762 and HR 1833 groups , both located in the OB1a subassociation , not far form the 25 Ori cluster .
We also confirm two stellar overdensities in OB1b , I and II , containing a total of 231 T Tauri stars .
A \sim 2 deg wide halo of young stars surrounds the Orion Nebula Cluster ; the north and south parts corresponding to the low-mass populations of NGC 1977 and NGC 1980 , respectively .
There is indication of two populations of young stars in the OB1b region , located at two different distances , which may be due to the OB1a subassociation overlapping on front of OB1b .
The various groups and regions can be ordered in an age sequence that agrees with the long standing picture of star formation starting in Orion OB1a some 10-15 Myr ago .

We use the strength of the H \alpha line in emission , combined with characteristics of IR excesses and optical variability , to define a new type of T Tauri star , the C/W class , stars we propose may be nearing the end of their accretion phase , in an evolutionary state between that of Classical T Tauri and Weak-lined T Tauri stars .
The evolution of the ensemble-wide equivalent width of the Li I \lambda 6707 line shows the depletion of Li with a timescale of 8.5 Myr .
The decline of the accretion fraction , from \sim 2 - 10 Myr , implies an accretion e-folding timescale of 2.1 Myr , consistent with previous studies .
Finally , we use the median amplitude of the V -band variability in each type of star , to show the decline of stellar activity , from the accreting Classical T Tauri stars to the population of least active field dwarfs .