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STIS Echelle observations at a resolution of 10 km/s and UVES/VLTspectroscopy at a resolution of 7 km/s of the luminous QSO HE 0515-4414 (z_em =1.73, B = 15.0) reveal four intervening O VI absorption systems in the redshiftrange 1.2 < z_abs < 1.7 (1.38503, 1.41601, 1.60175, 1.67359). In addition twoassociated systems at z = 1.69707 and z = 1.73585 are present. For the firsttime high resolution observations allow to measure radial velocities of H I, CIV and O VI simultaneously in several absorption systems (1.385, 1.674, 1.697)with the result that significant velocity differences (up to 18 km/s) areobserved between H I and O VI, while smaller differences (up to 5 km/s) areseen between C IV and O VI. We tentatively conclude that H I, O VI, and C IVare not formed in the same volumes and that therefore implications onionization mechanisms are not possible from observed column density ratios OVI/H I or O VI/C IV. The number density of O VI absorbers with W_rest > 25 mAis dN/dz < 10, roughly a factor of 5 less than what has been found by Tripp atal. (2000) at low redshift. An estimate of the cosmological mass-density of theO VI-phase yields Omega_b(O VI) = 0.0003 h^{-1}_{75} for [O/H] = -1 and anassumed ionization fraction O VI/O = 0.2. This corresponds to an increase byroughly a factor of 15 between z = 1.5 (this work) and the value found by Trippet al. (2000) at z = 0.21, if the same oxygen abundance [O/H] = -1 is assumed.Agreement with the simulations by Dave et al. (2001) can be obtained, if theoxygen abundance increases by a factor of 3 over the same redshift interval.Comment: 8 pages, 1 figure, accepted for publication in A&

High-resolution OVI absorption line observations at 1.2 $ \leq z \leq $ 1.7 in the bright QSO HE 0515-4414