This is going to be a quick post on how to disassemble and oil a computer case fan to reduce noise and increase fan longevity. All you need to take it apart and oil it is tweezers and WD-40. I am using a 50mm fan, but this guide is still relevant for 120mm and most other fans.
First thing to do is to remove the sticker on the back of the fan so that the bearing and O-ring are showing. There might be a cap to further protect the bearings, remove it.
Now take your tweezers and gently try to grab the white O-ring in there. It bends easily. Since mine was so small I had to use a very small flathead screwdriver to pry it out. Don't lose this piece once it's out. After it's out, lightly pull on the fan and it will come out.
Get the WD-40 ready. I recommend to use the one with a straw so you can aim. It's not a very big area so just get a little in there. You can also clean out the bearings with a napkin or something before the lubrication. I used canned air to blow away the dust and grime.
During reassembly, the O-ring will prove to be a problem. It might be easier with tweezers, but I had to use a T5 and T6 torx to push it back down. Also, if you rip the sticker, place some electrical tape over the hole.