There's a picture of the digital pinout in the RC in the bottom, the OI Port is explained below:
In the four ports, it works like that:
Pin 1 +5V Aux (Fuse F2) [2,3]
Pin 2 Joystick Trigger Switch
Pin 3 X-Axis
Pin 4 Ground
Pin 5
-- See below --
Pin 6 Y-Axis
Pin 7 Joystick Thumb Switch
Pin 8
-- See below --
Pin 9
-- See below --
Pin 10 Aux Switch1
Pin 11 Aux Analog
Pin 12 Ground
Pin 13 Wheel
Pin 14 Aux Switch2
Pin 15
-- See below --
Below -
Pins 5, 8, 9 and 15 are different for each port. For ports 1 and 3, they work as feedback LED's. For ports 2 and 4, they are the same as some switches in port 1 and 3. For example, pin 5 in port 2 is the same as pin 2 in port 3 (p3_sw_trig). If you need the pinout for those too, ask here.