X-ray absorption spectroscopy provides a potentially powerful tool in determining the metal abundances in various phases of the interstellar medium ( ISM ) . We present a case study of the sight line toward 4U 1820–303 ( Galactic coordinates l,b = 2 ^ { \circ } .79 , -7 ^ { \circ } .91 and distance = 7.6 kpc ) , based on Chandra Grating observations . The detection of O I , O II , O III , O VII , O VIII , and Ne IX K \alpha absorption lines allows us to measure the atomic column densities of the neutral , warm ionized , and hot phases of the ISM through much of the Galactic disk . The hot phase of the ISM accounts for about 6 % of the total oxygen column density \sim 8 \times 10 ^ { 17 } { ~ { } cm ^ { -2 } } along the sight line , with the remainder about evenly divided between the neutral and warm ionized phases . By comparing these measurements with the 21 cm hydrogen emission and with the pulsar dispersion measure along the same sight line , we estimate the mean oxygen abundances in the neutral and total ionized phases as 0.3 ( 0.2 , 0.6 ) and 2.2 ( 1.1 , 3.5 ) in units of Anders & Grevesse ( 1 ) solar value ( 90 % confidence intervals ) . This significant oxygen abundance difference is apparently a result of molecule/dust grain destruction and recent metal enrichment in the warm ionized and hot phases . We also measure the column density of neon from its absorption edge and obtain the Ne/O ratio of the neutral plus warm ionized gas as 2.1 ( 1.3 , 3.5 ) solar . Accounting for the expected oxygen contained in molecules and dust grains would reduce the Ne/O ratio by a factor of \sim 1.5 . From a joint-analysis of the O VII , O VIII , and Ne IX lines , we obtain the Ne/O abundance ratio of the hot phase as 1.4 ( 0.9 , 2.1 ) solar , which is not sensitive to the exact temperature distribution assumed in the absorption line modeling . These comparable ISM Ne/O ratios for the hot and cooler gas are thus considerably less than the value ( 2.85 \pm 0.07 ; 1 \sigma ) recently inferred from corona emission of solar-like stars ( Drake & Testa 2005 ) .