Simple. Disconnect oil pressure wire at sending unit on block. Turn on key. Only one light should come on. That is the charge light. Now go to the air cleaner switch by the air filter and short those two wires together. The light that comes on is the air cleaner light. The remaining light is the oil pressure light.
The diodes are to allow both the air cleaner light and the oil light to come on when the oil pressure switch closes. This serves as a bulb check for the air cleaner light.