[olddog]

Well it kind of depends on what you are installing and why.

If you are installing a wide band just to connect to a gauge to monitor AFR, say with a Carb, then definitely install it after the four pipes merge into one pipe, as far down as the side pipe will allow.

OEM EFI experts used to make a big deal out of the narrow band O2 should be like 15 - 20" from the head. One of the reasons is that OEM EFI systems are constantly testing the engine. It will lean a cylinder out and watch for the O2 to change. The distance to the O2 sensor impacts the response time. Also the closer the O2 to the engine, the quicker the O2 heats up to give a reading on a cold start. For these reasons and more, many experts recommended to put the O2 into a single pipe, rather than after the collector on long headers. Others claim they have figured out how to compensate for the longer distance, so it is only how fast the O2 heats up. If you have a good tune then it will be very close to the commanded AFR in open loop, so even that does not matter.

There is more, but I doubt you want to know, and I'm too lazy to type it all.