The output of an NPN switch does not have to go to the supply voltage. There is no need for additional opto-couplers or voltage dividers. A NPN output is simply just a switch to 0V.
Depending on the inputs to your BOB you have two simple options. If the input is connected to an opto-coupler then connect the anode to the BOB supply voltage (5V) and connect you proximity switch output to the cathode (may need to add a series resistor if BOB does not have one). If the BOB has a logic type input then simple connect the proximity switch output to the input. You may need to add a resistor between input and BOB supply (5V) if one does not already exist on the BOB.