The disclosure discloses a thin film transistor, a method for fabricating the same, and an OLED display panel. The method for fabricating a thin film transistor includes: forming a poly-silicon layer and a gate insulation layer on a base substrate in that order; forming a pattern of a gate above the base substrate with the gate insulation layer in a patterning process; doping the base substrate with the pattern of the gate for the first time; forming a pattern of first photoresist on the base substrate doped for the first time, using a mask for forming the pattern of the gate, wherein the pattern of the first photoresist covers the pattern of the gate and an area for forming a lightly doped drain area; and doping the base substrate with the pattern of the first photoresist for the second time to form a pattern of an active layer.