  We present optical/near-infrared photometry and spectra of the type Ia SN
2002bo spanning epochs from -13 days before maximum B-band light to +102 days
after. The pre-maximum optical coverage is particularly complete. In some
respects, SN 2002bo behaves as a typical "Branch normal" type Ia supernova (SN
Ia) at optical and IR wavelengths. We find a B-band risetime of 17.9+-0.5 days,
a Dm_{15}(B) of 1.13+-0.05, and a M_B=-19.41+-0.42. However, comparison with
other type Ia supernovae having similar Delta m_{15}(B) values indicates that
in other respects SN 2002bo is unusual. While the optical spectra of SN 2002bo
are very similar to those of SN 1984A, lower velocities and a generally more
structured appearance are found in SNe 1990N, 1994D and 1998bu. For supernovae
having Dm_(15)(B) > 1.2, we confirm the variation of R(SiII) (Nugent et al.
1995) with Dm_(15)(B). However, for supernovae such as SN2002bo, with lower
values of Dm_(15)(B) the relation breaks down. Moreover, the evolution of
R(SiII) for SN 2002bo is strikingly different from that shown by other type Ia
supernovae. The velocities of SN 2002bo and 1984A derived from SII 5640A, SiII
6355A and CaII H&K lines are either much higher and/or evolve differently from
those seen in other normal SNe Ia events. We suggest that the unusually low
temperature, the presence of high-velocity intermediate-mass elements and the
low abundance of carbon at early times indicates that burning to Si penetrated
to much higher layers than in more normal type Ia supernovae. This may be
indicative of a delayed-detonation explosion.
