We present a detailed study of the largest sample of intervening O vi systems in the redshift range 1.9 \leq z \leq 3.1 detected in high resolution ( R \sim 45,000 ) spectra of 18 bright QSOs observed with VLT/UVES .
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 ( \delta v ) of the O vi and C iv are strongly correlated ( at 5.3 \sigma level ) with \delta v ( \mbox { O { \sc vi } } ) being systematically larger than \delta v ( \mbox { C { \sc iv } } ) and ( iv ) \delta v ( \mbox { O { \sc vi } } ) and \delta v ( \mbox { C { \sc iv } } ) are also correlated ( at > 5 \sigma level ) with their respective column densities and with N ( H i ) ( 3 and 4.5 \sigma 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 4.3 \sigma level ) .
However , no significant correlation is found between N ( O vi ) and N ( H i ) .
These findings favor the idea that C iv and O vi absorption originate from different phases of a correlated structure and systems with large velocity spread are 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 \leq~ { } | \Delta v ( \mbox { O { \sc vi } } - \mbox { C { \sc iv } } ) |~ { } \leq 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 .
But a lack of systems with high N ( \mbox { O { \sc vi } } ) / N ( \mbox { H { \sc i } } ) ratio ( i.e. , \geq - 0.5 dex ) for z > 2.5 is noticeable .
Similar trend is also seen for the N ( \mbox { C { \sc iv } } ) / N ( \mbox { H { \sc 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 are 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 ) \Omega _ { \mbox { O { \sc vi } } } = ( 1.0 \pm 0.2 ) \times 10 ^ { -7 } for N ( \mbox { O { \sc vi } } ) > 10 ^ { 13.7 } cm ^ { -2 } , estimated in our high- z sample , is very similar to low- z estimations .