How do you turn down the temp of your oil heated hot water?

It would depend on how the water was heated. Tankless Coil, there should be a valve on the side of the coil, that has a knob on top, that is your mixing valve and should be used to adjust water temp. Indirect Hot Water Tank, it is connected to boiler through piping, but does not have it's own burner...should have a box on it, (aquastat), that will adjust the temp of your hot water, likewise with a Direct Fired Hot Water Tank, which is the same as mentioned above, but with it's own oil burner.