Not always can we pull an o-ring off the shelf in every size. With all the imported goods sold in the USA, we get things from everywhere, and as a result we get products with o-rings installed that may be metric, Japanese standard, or just a custom sized part. The solution to this is often finding the cord stock that is the right size and either "Cold Splicing" or "Hot Vulcanizing". Cold splicing is a good alternative in the field when you have to have something installed now. But to have the strongest joint, you want it hot vulcanized using the same rubber compound as the o-ring.
Hot Vulcanized O-rings are relatively easy to provide, and can generally be made and shipped in a day. Regardless of if it is Viton®, Neoprene, Nitrile, Silicone, EPDM or other synthetic rubber, this method works great for low volume orders