  Low-mass stars 0.1 ~< M ~< 1 Msun) in OB associations are key to addressing
some of the most fundamental problems in star formation. The low-mass stellar
populations of OB associations provide a snapshot of the fossil star-formation
record of giant molecular cloud complexes. Large scale surveys have identified
hundreds of members of nearby OB associations, and revealed that low-mass stars
exist wherever high-mass stars have recently formed. The spatial distribution
of low-mass members of OB associations demonstrate the existence of significant
substructure ("subgroups"). This "discretized" sequence of stellar groups is
consistent with an origin in short-lived parent molecular clouds within a Giant
Molecular Cloud Complex. The low-mass population in each subgroup within an OB
association exhibits little evidence for significant age spreads on time scales
of ~10 Myr or greater, in agreement with a scenario of rapid star formation and
cloud dissipation. The Initial Mass Function (IMF) of the stellar populations
in OB associations in the mass range 0.1 ~< M ~< 1 Msun is largely consistent
with the field IMF, and most low-mass pre-main sequence stars in the solar
vicinity are in OB associations. These findings agree with early suggestions
that the majority of stars in the Galaxy were born in OB associations. The most
recent work further suggests that a significant fraction of the stellar
population may have their origin in the more spread out regions of OB
associations, instead of all being born in dense clusters. (Abridged version).
