Your answer is in the Caution line. They say not to connect the serial DIRECTLY to the TTL bus. When you connect the OOPIC to the computer you use that serial port soldered to the PCB, right? That serial port is problably connect to a RS-232/TTL converter that is connected to the OOPIC chip's TTL port. However two of those pins soldered around the PCB are also connected DIRECTLY to the OOPICs TTL port. So, if you connect you decoder by using that serial port(the same one that you use to connect to the computer) then I am pretty sure it will be fine. But if you connect the decoder Output directly to the TTL pin the you will burn the controller. I have never used neither the oopic nor the decoder but I have seen already lots of parts that work similary, including my first micro controller.