An extrusion head (1) for blow moulding of hollow bodies comprises: a body (2) and a force plug (3) set within the body so as to define a cylindrical gap (4), through which the molten plastic material fed by an extruder can flow; a die opening (6) set at the bottom end of the body (2); a bottom part (30) of the force plug (3), set within the die opening (6) so as to define an annular gap (10) communicating with the cylindrical gap (4) for exit of the tubular parison; and a plunger (20) supporting at the bottom an ovalizing insert (40), slidably mounted within a chamber (32) formed in the bottom part (30) of the force plug. The plunger can pass from a retracted, resting, position, in which the bottom edge (48) of the ovalizing insert is located within the chamber (32), to an extracted, working, position, in which the bottom edge (48) of the ovalizing insert projects at the bottom with respect to the bottom edge (38) of the bottom part of the force plug.