OnePlus has always had to make strategic cuts to its phones in order to hit extremely enticing price points, and one of the handful of places it hasn't budged is in water resistance ratings — commonly seen as an "IPXX" number. As flagships have standardized on ratings like IP57 and IP68 across the board, OnePlus has stuck with its approach — and that will include the upcoming OnePlus 7 and OnePlus 7 Pro.
The company posted a short and typically snarky video explaining why: it just costs extra to get an IP certification. So instead, it saved some money by buying a bucket, filling it with water, and dunking a OnePlus 7. The inferred claim being that the OnePlus 7 series is water resistant like the other phones, but it simply hasn't been certified as such.
There's far more involved in IP ratings than just dropping a phone in a bucket of water, of course. IP, or ingress protection, certifies for both water and dust ingress to a device that affects its function under very specific circumstances. Different ratings certify different levels of protection, of course, but in general the testing will involve prolonged submersion in water, submersion at different depths, protection against powered water jets at various angles, and being exposed to dust particulates of various sizes. It costs money to get the certification, yes, but it's not all smoke and mirrors — there's actual testing involved.
So OnePlus could have paid to get a certification, but instead it bought some buckets and did its own testing. Testing that has yet to be explained or elaborated on, and with results that are implied but not confirmed. At the same time we don't know exactly how much money has been saved (for us or for OnePlus) by skipping the IP certification, but we can guess it's rather significant. Whether you think it would be worth the extra cash on a phone that's expected to retail for $600 or more is another question altogether.
Knowing it would be in deep water with the lawyer types if it actually claimed water resistance without the rating, its video has a common bit of fine print:
Products not IP certified. Water resistant under optimal test conditions. OnePlus makes no guarantees regarding water/liquid resistance. Water/liquid damage not covered under product warranty.
Even phones that have an IP rating typically don't warranty against water and dust damage, but OnePlus is obviously trying to walk a tight line here between inferring that the OnePlus 7 is water resistant and making sure that people don't go too far in trying to test its capabilities. Will the OnePlus 7 survive a splash or a trip to the beach? Probably. But so will most other phones that neither have an IP rating nor claim resistance. We just don't know what the functional difference between those phones and the OnePlus 7 actually is.
The bottom line is that if the OnePlus 7 and 7 Pro do not have an explicit IP rating, you're purely taking the company at its word rather than taking the word of a standardized testing mechanism that has been used across the industry for decades. Different people may trust a company at different levels, but I certainly will not be exposing the OnePlus 7 to water and dust with the same level of confidence I do a Galaxy S10. Perhaps that's a fine trade-off for saving a few hundred dollars.