Both differential scanning calorimetry and powder neutron diffraction have been applied to investigate an oxygen isotope effect on the metal-insulator (MI) transition in layered cobaltites RBaCo2O5.5 (R = Pr, Dy, Ho and Y). For all the compounds it was found that 18O substitution increases the transition temperature TMI by about 2 K. A small negative isotope-effect coefficient α0∼-0.06 indicates that a delocalization of the pd σ holes in the Co3+ high spin state (rather than a spin-state transition) can be responsible for the MI transition, in agreement with density-functional calculations (Wu 2003 J. Phys.: Condens. Matter 15 503).