  We present a new analysis of the stellar population of the Orion Nebula
Cluster (ONC) based on multi-band optical photometry and spectroscopy. We study
the color-color diagrams in BVI, plus a narrow-band filter centered at 6200A,
finding evidences that intrinsic color scales valid for main-sequence dwarfs
are incompatible with the ONC, while a better agreement is found employing
synthetic intrinsic colors obtained constraining the typical lower surface
gravity of young stars. We refine these model colors even further, empirically,
by comparison with a sample of ONC stars with no accretion and no extinction.
We consider the stars with known spectral types from the literature, and add 65
newly classified stars from slit spectroscopy and 182 M-type from narrow-band
photometry; in this way we isolate a sample of about 1000 stars with known
spectral type. We introduce a new method to self-consistently derive reddening
and accretion excess from the location of each star in the BVI color-color
diagram. This enables us to accurately determine the extinction of the ONC
members. We adopt a lower distance for the ONC than previously assumed, based
on recent parallax measurements. With a careful choice also of the spectral
type-temperature transformation, we produce the new H-R diagram of the ONC
population, more populated than previous works. With respect to previous works,
we find higher luminosity for late-type stars and a lower luminosity for early
types. We determine the age distribution of the population, peaking at 2-3 Myr,
a higher age than previously estimated. We study the distribution of the
members in the mass-age plane, and find that taking into account selection
effects due to incompleteness removes an apparent correlation between mass and
age. We derive the IMF for low- and intermediate-mass members of the ONC, which
turns out to be model-dependent, and shows a turn-over at ~<0.2Msun.
