The probability of positron 3gamma annihilation (I(3gamma)) due to the intrinsic decay of ortho-positronium (o-Ps) emitted from the surface of several porous oxide thin films was measured as a function of incident positron energy using a magnetically guided slow positron beam. The o-Ps diffusion length l(Ps) determined from I(3gamma) based on a simple one-dimensional model of diffusion was compared with the Knudsen diffusion length l(K) Of a classical particle with a mass of 2m(e). The good agreement between l(Ps) and l(K) for the sputter-deposited silicon-oxide films signifies that the Knudsen diffusion is a good approximation for describing the Ps diffusion in nanopores of the porous films. For the two spin-coated hydrogen-silsesquioxane films, l(Ps) of a film with higher porosity was about twice larger than the other film with less porosity, which means that o-Ps in the latter film has to travel slightly longer distance to come out from the film into vacuum.