Problems encountered during retrofitting of oil coolers in operating steam turbine units are discussed. Results obtained from numerical simulation of oil leaks occurring in the gaps between the tube bundles of oil coolers are presented. Different design versions of a modernized MB-270-330 oil cooler are calculated, and the effect of different design parameters on the performance characteristics of such apparatuses is analyzed. The adopted technical solutions made it possible to develop the oil cooler that has been installed in 13 of the 15 800-MW turbine units that are currently in operation in Russia.