I am trying to understand your question.

1. Heat Exchanger Configuration

- Counter-Current, Co-Current?

- What is flowing inside the inner pipe - oil or water?

- U should be defined for the diameter of the internal pipe.

2. Inlet and Outlet Temperature

- Are both known for water?

Typically, if the outlet temperature of oil is unknown, I would use the NTU method.

NTU = U.S/(M.Cp)_{min}

? = (M.Cp)_{min} / (M.Cp)_{max}

? = [ 1 - exp{ -NTU(1-?) } ] / [ 1 - ?.exp{ -NTU(1-?) } ] (for counter-current)

Q = ?(M.Cp)_{min}(T_{inlet,oil}-T_{inlet,water})

(M.Cp)_{oil} = F_{oil} * Cp_{oil}

(M.Cp)_{water} = F_{water} * Cp_{water}

where F is the mass flowrate and Cp is the specific heat capacity.

(M.Cp)_{min} refers to the lower of the 2 bulk heat capacities above.

(M.Cp)_{max} refers to the higher of the 2 bulk heat capacities above.

S refers to the contact surface area for heat exchange. Since this is a cylindrical heat exchanger, then S = Pi.D.L where D is the diameter of the internal pipe, and L is the length of internal pipe.

Once you had worked out the heat transfer rate Q, you can derive the actual outlet oil temperature from a simple heat balance, ie. Q = rate of heat gain by water = rate of heat loss by oil