You may open a profile and know the person. A colleague, a neighbour, a friend, someone from school. That happens on a local dating site. What you do next is the part that matters.
Keep it to yourself. Do not tell a mutual friend, do not mention it at work, and do not message them on another app to say you saw them. They joined a place that is not the open web. Outing them is not a funny story. It can cost them a relationship, a job, or simply the privacy they counted on.
You can close the profile and look at someone else. You can also send an ordinary message on Only Dating, about dating, if you would have wanted to anyway. What you cannot do is use the fact that you found them as leverage, gossip, or a joke.
The same rule protects you. Other members can see the username, age, town, and photos you chose to put here. They cannot see your surname unless you typed it in. A person who is not signed in cannot browse to the profile. The guide on who can see your profile is the exact list.
If someone does tell other people, or contacts you outside the site about what they saw, report the profile and block them. You do not have to explain yourself to them first.
If someone you know may already have an account, add each email address they use under Privacy in your account. You will not see a member who uses one of those addresses, and they will not see you, including if they join later. The site will not tell you whether an address is here. Once an address is added, it cannot be removed. That is so you cannot add it and take it away to see whether a profile appears. Check the spelling before you save it. You can add up to five. An address with a plus in it is treated as the address before the plus. name+notes@gmail.com is checked as name@gmail.com, including if that person joins later. You can also choose not to appear to other members in your own town, from the same Privacy page. You can still look through your town. They will not find your profile there.
Before you join, the men's page and the women's page both ask you to confirm this. It is a condition of using the site, not a polite extra.
Create a free account to message someone on Only Dating.