02 Nissan Altima Strut Assembly and O2 sensor

I need to replace the front right strut assembly on a friends Altima and could use a little help/advice.
Does the assembly come out as one piece with the spring? Then I would use a spring compressor while it is on the bench to compress, remove and put it into the new assembly.
If not, do I need to compress the spring while it is on the car?
Is this a very big job as it looks like 2 bolts to the wheel, 3 on top and one linkage?

Do I need to replace both sides at the same time? The reason I am replacing the right side is that the shalf seems to be bending in the middle when I push on the top of the wheel. I assume that something that holds the shaft steady is broken. The left side seems fine.

I also have a code for the bank 1 O2 sensor. Which sensor would I need to replace (top or bottom).

Thanks
Steve

I forgot to add the O2 code which is P0031. Am I correct in replacing the O2 sensor with that code and which sensor is bank 1?
Thanks
Steve

If I'm not mistaken, it should be the one right in you exhaust headers. Don't quote me on that.

I have replaced the struts. The only thing I should have done is loosen the top nut first before removing the strut.
I replaced the oxygen sensors but the light did not go off. Took it to the dealer who said the oxygen sensor were wrong and you need to get ones from the dealer.
Has anyone else found that the aftermarket oxygen sensors do not work?
Thanks
Steve