There are several ways you may go about suggesting treatment to people whom you believe have OCD.
The most direct approach is to just tell them that you have seen them performing some behaviors that appear to be rituals.
Let them know that you have read about OCD, that you think they may be exhibiting some symptoms, and that you’re willing to assist them in getting some help. Now, they may tell you that there is nothing wrong and that they have stopped doing whatever it is that you saw them doing. However, just let them know that you have noticed some things and they can come to you if they decide to pursue help.
You could also give them this book, telling them some of the behaviors they exhibit seem like something you saw in a movie or on a TV show, so you got this book to see if what they were doing was similar, and it looks as if it is.
If you are not comfortable talking face to face, you could write them a letter or an email and tell them you are concerned about them. You could even send them links to some of the websites that are cited in this book.
Finally, if they do decide to go to talk to a therapist, you could offer to go with them. Even if they do not want you to talk to the therapist, you could at least give them some moral support by driving to the appointment with them and talking to them afterward to be sure that they felt comfortable with everything.
Just keep in mind that the best time to talk to someone would be as soon as you think that the person has OCD. The sooner a person starts to work on his or her OCD, the fewer effects he or she will have from it.
Also, it will help to prevent the OCD from getting worse, therefore making the therapy potentially quicker.