The microstructures and formation mechanisms of various kinds of Ti(O,C) twins in a Nb–23Ti–15Al alloy have been investigated. The crystallographic features of the twins are systematically characterized using transmission electron microscopy (TEM). Lamellar or corner Ti(O,C) twins consist of coherent {111} twin boundaries (TBs) and incoherent {112} TBs. The three-fold Ti(O,C) twins are newly found, with symmetric Σ9 {114} or asymmetric Σ9 {115}/{111} serving as the secondary TBs. Moreover, slabs on Ti(O,C) TBs showing three-layer periodic contrasts are identified to be the twin overlapping. According to the microstructure characteristics, we propose a new model based on oriented attachment to interpret the formation of Ti(O,C) twins, which involves the contact, rotation, attachment and Ostwald ripening of Ti(O,C) grains. Our results provide new insights on the formation mechanisms of nanotwinned grains.