  Walraven photometry of established and probable members of the Orion OB1
association is presented. Effective temperature, surface gravity, luminosity
and mass are derived for all stars, using atmosphere models by Kurucz (1979).
Absolute magnitudes are calculated using the Straizys and Kuriliene (1981)
tables. Distance moduli and visual extinctions are determined. A comparison of
the visual extinctions to IRAS $100\um$ data shows that the near edge of the
Orion A and B clouds lies at a distance of $\sim 320\pc$, while the far edge is
at $\sim 500\pc$. A method for deriving the ages of the subgroups by comparing
theoretical isochrones to the observations in the log g, log T plane is
presented. The derived ages suggest, contrary to earlier studies, that subgroup
1b is younger than 1c, which can possibly be explained by past geometries of
the system of stars and gas. The initial mass function for Orion OB1 is derived
with the aid of the Kolmogorov-Smirnoff test. Through extensive simulations, we
show that it is very difficult to derive accurately the IMF from the available
data. To within somewhat weak limits the IMF is found to be of the form $\xi
(\log M)=AM^{-1.7\pm 0.2}$ for all subgroups. The energy output of the
subgroups in the form of stellar winds and supernovae is calculated and
compared to the observed size and expansion velocity of the Orion-Eridanus
bubble. It is shown that the energy output of the association can account for
the morphology and kinematics of the ISM.
