The O ring is made of rubber and its size is very small. It can be locked in the connection between the dart steam and the dart barrel or dart flight, that as a fastener so that when a player shooting the dart at dartboard, the parts of the dart will not come loose.
When doing the installation, you better put a solid color paper on the table where you are operating, Once you accidentally slide the O ring from your dart steam, you can find it easily. you can do the installation by hand, but because the O ring is too small, it is better to use an O ring tool, we can put the O ring to the dart steam firstly, and then slide the O ring from the tip end to the aim position, that will easy and will not fall out, that make the replacement easier.