Disclosed herein are oligomerization processes in which ethylene and a catalyst system are first combined for a suitable residence time in an activation vessel, prior to introduction into a reaction zone to oligomerize ethylene to form a desired oligomer product, such as 1-hexene and/or 1-octene. Related oligomerization reaction systems that include the activation vessel also are disclosed. In these oligomerization processes and reaction systems, the catalyst system can be fully activated as it leaves the activation vessel and enters the reaction zone, thus providing greater catalyst utilization and less catalyst waste.