A process is described for carrying out olefin oligomerisation in two catalysis steps of different types, using an organic feed containing at least one olefin. The cut to be treated, containing at least one olefin (C.sub.n), is introduced into a first reaction zone where, in a first step, it undergoes catalytic oligomerisation either of the homogeneous liquid phase type, or of the heterogeneous type with a solid support. The effluent produced is generally sent to a heat exchanger traversed by a cold liquid, the effluent thus being cooled before being sent to a second reaction zone where it undergoes catalytic oligomerisation in a liquid--liquid two-phase medium, the invention being characterized in that at least an ethylenic hydrocarbon containing 3 to 5 carbon atoms per molecule is added to the inlet to the second reaction zone.