Using high-resolution ultraviolet spectra of 16 low-z QSOs obtained with the E140M echelle mode of the Space Telescope Imaging Spectrograph, we study the physical conditions and statistics of OVI absorption in the intergalactic medium (IGM) at z<0.5. We identify 51 intervening (z_abs_<z_QSO_) OVI systems comprising 77 individual components, and we find 14 "proximate" systems (z_abs_~z_QSO_) containing 34 components. For intervening systems (components) with rest-frame equivalent width W_r_>30m{AA}, the number of OVI absorbers per unit redshift dN/dz=15.6^+2.9^_-2.4_(21.0^+3.2^_-2.8_), and this decreases to dN/dz=0.9^+1.0^_-0.5_(0.3^+0.7^_-0.3_) for W_r_>300m{AA}. The number per redshift increases steeply as z_abs_ approaches z_QSO_; we find that dN/dz is ~3-10 times higher within 2500km/s of z_QSO_. The most striking difference between intervening and proximate systems is that some proximate absorbers have substantially lower HI/OVI ratios.