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We investigate the impact of optically thick clumping on stellar winddiagnostics in O supergiants and constrain wind parameters associated withporosity in velocity space. This is the first time the effects of opticallythick clumping have been investigated for a sample of massive hot stars, usingmodels including a full optically thick clumping description. We re-analysespectroscopic observations of a sample of eight O supergiants. Using a geneticalgorithm wrapper around the NLTE atmosphere code FASTWIND we obtainsimultaneous fits to optical and UV spectra and determine photospheric and windproperties and surface abundances. We provide empirical constraints on a numberof wind parameters including the clumping factors, mass-loss rates and terminalwind velocities. Additionally, we establish the first systematic empiricalconstraints on velocity filling factors and interclump densities. Theseparameters describe clump distribution in velocity-space and density of theinterclump medium in physical-space, respectively. We observe a mass-loss ratereduction of a factor of 3.6 compared to theoretical predictions from Vink etal. (2000), and mass-loss rates within a factor 1.4 of predictions fromBj\"orklund et al. (2021). We confirm that including optically thick clumpingallows simultaneous fitting of recombination lines and resonance lines,including the unsaturated UV phosphorus lines (Pv 1118-1128), without reducingthe phosphorus abundance. We find that, on average, half of the wind velocityfield is covered by dense clumps. We also find that these clumps are 25 timesdenser than the average wind, and that the interclump medium is 3-10 times lessdense than the mean wind. The former result agrees well with theoreticalpredictions, the latter suggests that lateral filling-in of radially compressedgas might be critical for setting the scale of the rarefied interclump matter.

Empirical mass-loss rates and clumping properties of Galactic early-type O supergiants