The startup of a tubular reactor containing fresh or regenerated molecular sieve catalyst and cooled by steam generation on the shell side, as part of an olefin oligomerization process, is improved by, during the startup phase of the reactor, controlling the pressure on the shell side of the reactor at maximum 10 barg. The startup may be further improved by controlling the inlet temperature of the hydrocarbon startup stream to the reactor, by controlling the flow of reactant olefins to the reactor, or by controlling the nature and/or concentration of the olefins in the reactor feed.