  We report the first detection of an OVIII Ly-alpha absorption line associated
with an overdense region in the intergalactic medium (IGM) along the sightline
towards PKS 2155-304 with the Chandra Low Energy Transmission Grating
Spectrometer (LETGS). The absorption line is detected at 4.5 sigma level with
cz~16,600 km/s. At the same velocity Shull et al.(1998) detected a small group
of spiral galaxies (with an overdensity of delta_gal ~ 100) and low metallicity
HI Ly-alpha clouds. We constrain the intragroup gas that gives rise to the
OVIII Ly-alpha line to a baryon density in the range 1e-5 < n_b < 7.5e-5 cm^-3
(50 < delta_b < 350) and a temperature of (4-5) x 10^6 K, assuming 0.1 solar
abundance. These estimates are in accordance with those of the warm/hot
intergalactic medium (WHIM) that are predicted from hydrodynamic simulations.
Extrapolating from this single detection implies a large fraction of the
``missing baryons'' (~ 10%, or ~ 30-40% of the WHIM) are probed by the OVIII
absorber.
