I had a friend with OCD. One of the most common examples they showed was when putting on a hoodie, they were turn it inside out and back. Doing this 3-4 times every time they put wanted to put it on. There would be occasions where the ritual didn't 'work' and they'd keep doing it. If i were there they'd ask me to reassure them they wouldn't die if they stopped and put it on.
Logically they knew it made no sense, but they were unable to convince themselves of it.
Other issues could last anywhere from a few minutes to a few days. And for people of a religious persuasion it can often add more problems for them. My friend, for example, didn't believe someone could lose their salvation, yet their OCD would kick in and they would be in a near panic believing they lost theirs. Mostly trying to discuss it with them, to rationalize it, didn't work, and sometimes made it worse. At times this lasted days, causing great anxiety in them.
In some cases you may have to avoid certain types of discussions even. Merely mentioning things could trigger them. It's very much a 'let them handle it themselves, unless they ask for help'. And even if they ask for help, they usually can only handle it in short doses before telling you to stop. And if that's the case, stop immediately.
Typically your goal is to help them after an episode, to calm down, not help them through the episode, where you'll make it worse.