Oil in radiator& reservoir?

I have a 2001 Honda Civic LX. 5 months ago, I had the head gasket, timing belt, water pump, 16 valves, thermostat, spark plugs, crank shaft seal (2x), oil pan, oil pump, exhaust manifold and a crank angle sensor replace. Now, I have oil in my radiator and reservoir. Did the mechanic forget something or can a head gasket blow up in 5 months?

Wow, you had a lot replaced on a not so old car. That must have cost a fortune and something tells me it was not even necessary to replace All those things.
Did he also replace intake manifold gasket?
Oil in the water surely points to a gasket issue. Do you also have water in your oil? Does your car sometimes start up pretty rough?
The mechanic probably didn't do something right because there's no way that wore out so quickly. Sorry to hear about your troubles too!