The free volume hole fraction for ortho-terphenyl (OTP) in the liquid state above the glass transition have been determined by two different approaches. The first, experimental approach consists in analysis of the ortho-positronium (o-Ps) annihilation parameters of OTP by means of our recent phenomenological one phase model of volumetric and free volume hole properties. The second one is based on detailed atomistic simulations of bulk OTP systems via energy minimization and subsequent rigid probe insertion analysis of the minimized systems. We have found a qualitative consistency between both the free volume hole fraction vs temperature courses supporting the opinion that the o-Ps feels the free volume microstructure of the liquid state of OTP, at least.