Metastable beta titanium alloys with increased oxygen content represent an attractive, but not yet fully understood area of titanium alloys. Their main advantage over conventional alloy Ti-6Al-4V used for manufacturing of joint implants is high strength and low modulus of elasticity. In this work, we aimed to study the influence of oxygen and beta-stabilizing elements on the crystal structure of the martensitic phase arising during quenching or deformation. The main emphasis was given to the investigation of the structure parameters in the parent beta phase depending on a wide range of oxygen concentrations. In the studied Ti-Nb-O systems, the proportion of Nb and O was optimized to achieve the formation of the martensitic phase. Their presence was detected by SEM. Structural parameters were measured by X-ray diffraction using the 2D detector.