Clustering and 10 Myr old stellar aggregates in the Orion OB1 Association

I will present the results of a photometric and spectroscopic census of the Orion OB1 association, spanning ~180 sq.deg., with emphasis on the off-cloud regions containing somewhat older populations of stars in the age range ~4-10 Myr. With ~2000 confirmed T Tauri stars, this is the largest, most complete sample of low-mass young stars in this important evolutionary stage. We show that the off-cloud stellar population consists of various newly recognized clusters amidst a general low-density background, and that the Orion Nebula Cluster is surrounded by a few sq.deg halo of young stars