An oilhead is the name used for BMW motorcycle engines that advertise the use of oil cooling to transfer some of the heat from the engine to the atmosphere.
The concept of oil-cooling engines is not revolutionary or even new, since engine oil has always had the effect of carrying away heat, usually for it to be dissipated to the atmosphere via a sheet steel or finned cast-alloy sump or rocker box. But modern variations (eg Suzuki's GSX-R 750) employ an oil-radiator to cool the oil, and the forms of circulation introduced encourage the engine oil to heat up more quickly with benefits to engine life, economy and perhaps performance.