Many of the high-end smartphones nowadays come with water-resistance protection. The OnePlus 5 however lacks water resistance. I'm sure that many users expected this flagship phone to have a certified Intentional Protection Marking based on the IEC standard 60529 like many of the other premium phones released in 2017.
If you search for the term 'OnePlus 5 water resistance', you probably going to see some articles and videos show that the OnePlus 5 can withstand submersion in water and still survive. That lead people to assume that the phone is indeed waterproof and OnePlus just didn't mentioned it in the launch event or on their website.
Just so you know, that even if you see the OnePlus 5 surviving a water test in many of those videos, it doesn't have any certification for that. Just for example, the Samsung Galaxy S8 and S8+ have IP68 rating based on the standards that I mentioned above. They have been certified for being water-resistant and you can put them in fresh water to a maximum depth of 1.5 meters for up to 30 minutes and no harm will be done to the phone. They also have protection from dust as well. Of course there are some limitations even for that, like not exposing the phone to water moving with force, don't try to submerge the phone deeper to what the IP-rating indicates and know what some of the functionality of the phone might be effected when used under water, like the touch screen won't respond properly. Furthermore, the phone might stop working even hours after those test was done. You usually see people just putting the phone underwater and testing it for a short time and see if it works when they take it out.
Here is just one video posted by Techno Unboxing. He put the OnePlus 5 to a water test by submerging it into a bowl full of water for one minutes and then testing if the phone works, checking the speaker, etc. As you can see in that particular video, the OnePlus 5 seems to survive its small water-resistance test.
Of course you should not try it yourself as this can lead to a malfunction of the device and the device itself isn't certified for that type of underwater use.