An improved olefin upgrading technique has been developed for increasing production of premium heavy hydrocarbons, such as distillate fuel, from lower olefinic feedstock. During recovery and recycle of intermediate range hydrocarbons products, a process has been found for withdrawing a fraction rich in C.sub.5 -C.sub.9 gasoline range olefinic hydrocarbons from the oligomerization reactor effluent stream. By separating the reaction effluent in a multistage distillation system, fractionation feed can be separated into a heavier bottoms stream rich in C.sub.10+ hydrocarbons and a light hydrocarbon overhead, while withdrawing a liquid stream as an overflash fractionation stream rich in intermediate hydrocarbons. The overflash stream is combined to form a portion of the recycle stream to the reactor, thereby providing a more efficient and lower cost recovery process.