No, it will not do a thing to the OBC. The only problem you may run into is the factory amp. There are ways to get around it, but others here can post more detailed instructions on that.
To install the actual HU, you need to remove the factory one. There are two small flaps on the sides. Behind them are 8mm hex nuts. Use an allen key and remove them. The radio can then be pulled out. Disconnect the battery if you want to avoid the beeping noise of the anti-theft device. Pull off the harness and your almost done.
Take the two wiring harnesess you have (the one that came with the HU and the one you will buy as an adapter for the car) and connect the wires. You sodder, butt connect, spade connect, or just tape the connections its up to you. I prefer uninsulated buttconnectors covered with paint-on tape. Slide the trim ring into place (also will need to purchased seperately) behind the mounting sleeve, and push the sleeve into place. Bend the tabs with a screwdriver to hold them it place. Connect your harnesses, slide the HU into place, when it clicks attatch the face plate. YOUR DONE!
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>