Living with or even having to see someone with OCD can be tough, but you have to remember that they are people and didn't choose to be like that.

[edit] Steps

First, you have to be really patient with the person in general.
Talk to them. If you feel really bothered by the way they behave, you want to tell them very carefully and with caution that they are taking things too seriously and that you know they can't control it but that they are hurting others by paying more attention to something they don't even know why they are obsessing over.
Get a mediator. If they don't listen to you and become aggravated, ask someone else to do it, because then the more people talking to the person, the more motivated the person will feel about how they are letting others down. Suggest something to them, like a therapist or a medication of some sort.
Take their mind off the obsession. We usually like to bother my mom to take us somewhere, like the mall, so that she will put her Swiffer down and go have something to do with her kids.