We investigate the origin and physical properties of O VI absorbers at low redshift (z= 0.25) using a subset of cosmological, hydrodynamical simulations from the OverWhelmingly Large Simulations (OWLS) project. Intervening O VI absorbers are believed to trace shock-heated gas in the warm-hot intergalactic medium (WHIM) and may thus play a key role in the search for the missing baryons in the present-day Universe. When compared to observations, the predicted distributions of the different O VI line parameters (column density ?, Doppler parameter ?, rest equivalent width Wr) from our simulations exhibit a lack of strong O VI absorbers, a discrepancy that has also been found by Oppenheimer & Davé. This suggests that physical processes on subgrid scales (e.g. turbulence) may strongly influence the observed properties of O VI systems. We find that the intervening O VI absorption arises mainly in highly metal enriched (10-1≪Z/Z⊙≲ 1) gas at typical overdensities of 1 ≪ρ/<ρ>≲ 102. One-third of the O VI absorbers in our simulation are found to trace gas at temperatures T < 105 K, while the rest arises in gas at higher temperatures, most of them around T= 105.3 ± 0.5 K. These temperatures are much higher than inferred by Oppenheimer & Davé, probably because that work did not take the suppression of metal-line cooling by the photoionizing background radiation into account. While the O VI resides in a similar region of (ρ, T)-space as much of the shock-heated baryonic matter, the vast majority of this gas has a lower metal content and does not give rise to detectable O VI absorption. As a consequence of the patchy metal distribution, O VI absorbers in our simulations trace only a very small fraction of the cosmic baryons (<2 per cent) and the cosmic metals. Instead, these systems presumably trace previously shock-heated, metal-rich material from galactic winds that is now mixing with the ambient gas and cooling. The common approach of comparing O VI and H I column densities to estimate the physical conditions in intervening absorbers from QSO observations may be misleading, as most of the H I (and most of the gas mass) is not physically connected with the high-metallicity patches that give rise to the O VI absorption.