Is there any code prohibition that speaks against locating an electric
hot water heater close to heating oil tanks?

I have an oil fired hot water heater in the mechanical room at one end
of my basement and would like to place an electric water heater below
the middle of the house (about 100 ft from the oil fired water heater,)
nearer to guest rooms to make sure that when we have guests we don't run
out of hot water for them. I have a Ford gravity aided recirculating
loop and plan to set the thermostat on the electric heater below the
temperature of the water supplied by the oil fired heater so the
electric heater should not be active unless we have a high demand for
hot water.

My concern about location is because I have 3, 330 gallon oil tanks
right next to where I'd like to locate the electric water heater.

Boden