To make the power / system status LED work I had to remove the D8 LED on the OPI board. I connected a NPN transistor's base to the positive end of D8 and the collector to the 5v line. The emitter goes to ground through a green LED and a 220 ohm resistor.
For the SD activity LED I connected the /CS line of the SD slot (pin 2) to the base of a PNP transistor. The collector of the transistor is connected to the 5v line through a red LED and a 220 ohm resistor. The emitter is connected to ground. This inverts the signal from the /CS line making the LED blink when /CS goes low and the SD card is being accessed by the OPI
Now when you boot the OPI you will first see the SD activity LED blink a few times, presumably to fetch the boot loader. When everything is ok the green status LED wit turn on and the OPI will continue booting and the SD LED will blink some more like you would expect :).