You will always find that non-manufacturer branded parts that are a one-to-one replacement are cheaper. Does not mean worse quality. I will admit though, I had a couple of 3rd-party replacement parts in the past that were just slightly off of original spec, even though they were supposed to be an exact fit. In the end, they had to be replaced, and I had to go with original manufacturer parts. Had to pay twice basically. One was the tensioner pulley. 3rd-party part was off by a small amount. It ended up throwing belts just part way until they would split and shred. It wasn't until I replaced it with factory parts that I saw the slight difference.
Just one example. I am sure that this is the exception to what most people will find, but I thought that I would share.
In the case of O2 sensors, I am sure that the Bosch ones are OK. Don't let the price fool you. I just wish that I had a machanic in town that I could trust that didn't always have to put manufacturer parts in. I could have saves $130 and still had them do it.
You will find that a lot of times, one company will make parts for another and their own branded items are less expensive. In the automotive industry, it just seems more noticeable, but I have seen it in Audio/Video, and other places I'm sure, but just don't realize it.