A process and related reactor (1) for oxidation of a hydrocarbon feedstock are disclosed, the reactor (1) comprising a vessel (3) and a neck (5) with an axial burner (6) and a tangential gas inlet (2), wherein the neck (5) has a swirling chamber (10) located below said burner (6) and connected to said gas inlet (2), to produce a gas vortex (V) which optimizes the mixing between the gas stream (G) and the oxidizer in said neck (5). Preferably the swirling chamber (10) has an internal surface (12) with a log-spiral profile.