First, you are most likely already interacting with someone with OCD in your daily life without even being aware of it.
There are so many people with OCD that it would be surprising if you did not know someone with the disorder.
This person may be able to control his or her OCD enough at work or in social situations that you don’t even notice it, or maybe it’s manifested through mostly obsessions and mental rituals, so you never actually see the person performing any overt behavioral rituals.
If you do meet someone with OCD, and it’s either apparent or the person tells you he or she has it, the best thing to do would be to treat the person, hopefully, like you would treat everyone else, with respect and compassion. People with OCD did not ask to have the disorder and desperately want to be rid of it, but they are facing fears that are very real to them, so they need support to get through their difficulties.
That said, do not give a lot of reassurance to people with OCD. It may seem like a compassionate thing to do, but people with OCD are looking for reassurance, and you may unwittingly play right into their desire to attain immediate relief and feelings of safety.
It is fine to encourage them to follow through on their therapy and challenge their fears, but it is a slippery slope when you give them reassurance, which will actually help to maintain their OCD.