The pseudobinary Zr1Nb–O phase diagram was recently estimated at the Czech research institute UJP using a new experimental procedure. It is based on the oxygen distribution measurement inside fuel claddings previously exposed to high-temperature steam environments, assuming equilibrium conditions being fulfilled at the phase boundaries. The experimental results agreed satisfactorily with the CALPHAD approach, and a further experimental program followed to affirm the phase diagram. This paper is concerned with additional measurements of the oxygen concentrations using wavelength-dispersive spectrometry, secondary ion mass spectrometry, and inert gas fusion techniques and examination of the microstructure in the Zr1Nb cladding wall after high-temperature oxidation. The main goal was to check the validity of the recently proposed Zr1Nb–O phase diagram, or to refine it. The new results of the oxygen concentration measurements confirmed the phase diagram. After that, a new diffusion model, the Jakub Krejci oxidation model, was created using the Zr1Nb–O phase diagram describing the double-sided high-temperature oxidation of the Zr1Nb fuel cladding including the (α + β)-Zr layer. The numerical calculations were compared to the experimental results with satisfactory agreement. It could be concluded that the proposed experimental procedure provided a good estimation of the Zr1Nb–O phase diagram. Moreover, it can be used for alloys containing a greater amount of hydrogen. The Zr1Nb–O phase diagram may be applicable for oxygen diffusion models predicting the oxidation behavior of Zr1Nb fuel claddings upon high-temperature oxidation.