Intergalactic absorbers along lines of sight to distant quasars are a powerful diagnostic for the evolution and content of the intergalactic medium ( IGM ) . In this study , we use the FUSE satellite to search 129 known Ly \alpha absorption systems at z < 0.15 toward 31 AGN for corresponding absorption from higher Lyman lines and the important metal ions O vi and C iii . We detect O vi in 40 systems over a smaller range of column density ( log N _ { OVI } = 13.0–14.35 ) than seen in H i ( log N _ { HI } = 13.0–16.0 ) . The co-existence of O vi and H i suggests a multiphase IGM , with warm photoionized and hot ionized components . With improved O vi detection statistics , we find a steep distribution in O vi column density , d { \cal N } _ { OVI } / dN _ { OVI } \propto N _ { OVI } ^ { -2.2 \pm 0.1 } , suggesting that numerous weak O vi absorbers contain baryonic mass comparable to the rare strong absorbers . Down to 30 mÅ equivalent width ( O vi \lambda 1032 ) we find an absorber frequency d { \cal N } _ { OVI } / dz \approx 17 \pm 3 . The total cosmological mass fraction in this hot gas is at least \Omega _ { WHIM } = ( 0.0022 \pm 0.0003 ) [ h _ { 70 } ( Z _ { O } / 0.1 Z _ { \odot } ) ( f _ { OVI% } / 0.2 ) ] ^ { -1 } where we have scaled to fiducial values of oxygen metallicity , O vi ionization fraction , and Hubble constant . Gas in the WHIM at 10 ^ { 5 - 6 } K contributes at least 4.8 \pm 0.9 % of the total baryonic mass at z < 0.15 . We then combine empirical scaling relations for the observed “ multiphase ratio ” , N _ { HI } / N _ { OVI } \propto N _ { HI } ^ { 0.9 \pm 0.1 } , and for hydrogen overdensity in cosmological simulations , N _ { HI } \propto \delta _ { H } ^ { 0.7 } , with the H I photoionization correction to derive the mean oxygen metallicity , Z _ { O } \approx ( 0.09 Z _ { \odot } ) ( f _ { OVI } / 0.2 ) ^ { -1 } in the low- z multiphase gas . Given the spread in the empirical relations and in f _ { OVI } , the baryon content in the O vi WHIM could be as large as 10 % . Our survey is based on a large improvement in the number of O vi absorbers ( 40 vs. 10 ) and total redshift pathlength ( \Delta z \approx 2.2 vs . \Delta z \approx 0.5 ) compared to earlier surveys .