loading on rail voltages in OPAMP
Most op-amps should have a spec for supply current in the datasheet. This current flows from the positive supply pin to the negative supply pin, so it loads both of your supplies.

In addition you need to consider the current flowing from the op-amp's output pin into the load and the feedback network. If the op-amp is sourcing current, the output pin current comes from the positive supply, and loads only that supply. If the op-amp is sinking current, the output pin current flows to the negative supply, and loads only that supply.

You need to determine the output pin current by considering the operating behavior of the circuit, the load impedance, and the current that will be drawn by the feedback network.