  We present results of our large scale, optical, multi-epoch photometric
survey of ~180 square degrees across the Orion OB1 association, complemented
with extensive follow up spectroscopy. We map and characterize 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% in OB1b, and 14% 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, HD 35762
and HR 1833 in the OB1a subassociation, and two stellar overdensities in OB1b.
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 define a new type of T Tauri star, the C/W class, objects we propose may
be nearing the end of their accretion phase. We detect the observational
signature of Li depletion in young K and M stars with a timescale of 8.5 Myr.
The decline of the accretion fraction from ~2 - 10 Myr, implies an accretion
e-folding timescale of 2.1 Myr. Finally, the median amplitude of the V-band
variability shows the decline of stellar activity, from accreting Classical T
Tauri stars to the least active field dwarfs.
