  We review the current knowledge of the population of `older' stars in the
Orion OB1 association, specifically those in subgroups 1a and 1b. We briefly
outline the history of the subject and then continue with a summary of the
present state of knowledge of the early-type stars in Orion OB1. New results
from the Hipparcos parallaxes and proper motions will be presented. The main
result is that subgroup 1a is located at about 330 pc from the Sun, much closer
than the previously determined distance, and about 100 pc distant from the
other subgroups of the association and the Orion molecular clouds.
Unfortunately, due to the unfavorable kinematics of the association with
respect to the Galactic background, Hipparcos proper motions do not allow a
clear kinematic separation of the association from the field. For this purpose
accurate and homogeneous radial velocities are needed. Traditionally, the
massive O and B stars have received most of the attention in the studies of OB
associations. However, we will present results showing that significant numbers
of low-mass stars are associated with Orion OB1. Unbiased, optically complete,
spectroscopic and photometric surveys of areas within subgroups 1a and 1b have
the potential to determine the complete low-mass stellar population, down to
the brown dwarf limit. This will provide much insight into the overall initial
mass function and studies of the kinematics of the low-mass stars will yield
insights into the dispersal of the association.
