“Or-Gate” in Minecraft

Install an output-redstone-lane. In my case I am using a sticky piston with a gold block on top to indicate the state of the output signal.
oder_gatter-1

Build another two redstone-lanes which should form a ‘cross’ with the existing output-lane.
oder_gatter-2

Place 3 levers at the ends of the lanes.
Explanation:
At least one lane needs to be powered to produce a positive output.