The bottom line as we know is that ultimately the air flow has to carry all the heat away from the installation.  So oil to water heat exchangers end up needing (as Ken points out) a larger radiator (possibly 1/3 larger) to handle the additional heat dumped into the coolant by the oil/water exchanger.. 

Just an added comment, the amount of heat exchanger volume added to the coolant rad is more than would be needed for the direct to air oil cooler for two reasons: the temp drop from oil to water, and the upper temp limit on the coolant is generally lower than on the oil; both resulting in lower temp for rejecting the heat – therefore bigger volume.  If you have the room; the additional volume is less expensive, and may not weigh as much, because of the lower pressure and viscosity.

I believe that oil/water heat exchangers might be justified in an aircraft installation if for some reason the lay-out precluded getting air to an oil cooler but did permit you to install a larger radiator capable of handling the heat load for the coolant and oil.  Otherwise, I don't believe they would work very well in most of our installations.  There could be exceptions of course.

My $0.02

Ed Anderson