We present a detailed study of the largest sample of intervening O vi systems in the redshift range 1.9 ≤z≤ 3.1 detected in high-resolution (R∼ 45 000) spectra of 18 bright quasi-stellar objects observed with Very Large Telescope/Ultraviolet and Visible Echelle Spectrograph. Based on Voigt profile and apparent optical depth analysis we find that (i) the Doppler parameters of the O vi absorption are usually broader than those of C iv, (ii) the column density distribution of O vi is steeper than that of C iv, (iii) line spread (δv) of the O vi and C iv is strongly correlated (at 5.3σ level) with δv(O vi) being systematically larger than δv(C iv) and (iv) δv(O vi) and δv(C iv) are also correlated (at >5σ level) with their respective column densities and with N(H i) (3 and 4.5σ, respectively). The median column densities of H i, O vi and C iv are found to be higher when low ions are present. N(C iv) and N(H i) are strongly correlated (at 4.3σ level). However, no significant correlation is found between N(O vi) and N(H i). These findings favour the idea that C iv and O vi absorption originate from different phases of a correlated structure and systems with large velocity spread is probably associated with overdense regions. The velocity offset between optical depth weighted redshifts of C iv and O vi absorption is found to be in the range 0 ≤|Δv(O vi–C iv)|≤ 48 km s−1 with a median value of 8 km s−1.
We do not find any evidence for the ratios N(O vi)/N(H i), N(O vi)/N(C iv) and N(C iv)/N(H i) to evolve with z over the redshift range considered here. However, a lack of systems with high N(O vi)/N(H i) ratio (i.e. ≥−0.5 dex) for z > 2.5 is noticeable. Similar trend is also seen for the N(C iv)/N(H i) ratio. We compare the properties of O vi systems in our sample with that of low-redshift (z < 0.5) samples from the literature and find that (i) the O vi components at low z are systematically wider than at high z with an enhanced non-thermal contribution to their b parameter, (ii) the slope of the column density distribution functions for high and low z is consistent, (iii) the range in gas temperature estimated from a subsample of well-aligned absorbers is similar at both high and low z and (iv) for N(O vi) > 1013.7 cm−2, estimated in our high-z sample, is very similar to low-z estimations.