<p>The cooling of Ti₂AlNb alloy from the B2 phase region was interrupted at different temperatures to investigate the microstructure evolution. Two nucleation approaches of grain boundary O phase (O_GB) were discovered. O_GB usually keeps a special orientation relation (orthorhombic-BCC orientation relation, OBOR) with the parent B2 grain and the variant selection follows the criterion that the {001}_OGB pole is parallel to closest {110}_B2 poles of adjacent B2 grains, when the angle <i>θ</i> between the closet {110}_B2 planes is less than 5&#xa0;deg. The Widmanstätten O (O_WGB) can nucleate from the prior <i>α</i>_2GB, O_GB, or directly form from the B2 grain boundaries. After initial burst of nucleation to create the skeleton templates of the O_WGB colony, the further transformation is proceeded to infill the B2 grain through two mechanisms, <i>i.e.</i>, branching or broadening of the primary O_WGB laths. Eventually, the formed O_WGB colony has large area parallel side plates whose crystallographic orientations are similar to each other. The O_WGB usually inherits the crystallographic orientation of the parent phase and all the O_WGB keep the OBOR with its host B2 grain. These results highly deepen our understanding of the precipitation of O phase in Ti₂AlNb alloy.</p>