How can you measure the interface between oil and water in a floating roof tank (or open tank, doesn't matter) using DP transmitter?

take a sample from the top (where the oil live there) and from the bottom (where water stay) to DP.
because of the constant distance between two samples point , so the only variable there is the hydrostatic pressure according to interface movements.

we use displacement type levetroll(fischer) for interface level measurements ,using DPT i doubt measuring interface level in tank(used where head measurements applicable)

Interface measurements are not reliable using a DPT, a multivariable DPT is better option, also RF Capacitance is best and economic option

We can measure DP type meter. you calculate lower density liquid of100% is4mA. and Higher density liquid100% is20mA. at dp transmitter. 

No way to measure it with a DP Transmitter