I'm happy to hear about an improved telephoto, since a quality telephoto is one of the reasons I bought the OnePlus 12 last year.
I was hoping the new model would have a telephoto similar to the Oppo Find X8 Ultra or the vivo X200 Ultra, but the Oppo has a 70mm f/2.1 with a large 1/1.56" sensor, and the vivo has an 85mm f/2.3 with a slightly larger 1/1.4" sensor. If the OnePlus 15 has an f/2.8 lens, then it's going to be at a disadvantage to those other two phones, unless it has a larger sensor, which is probably too much to hope for. However, the Sony LYT-818 sensor, which vivo uses for some of its cameras, is 1/1.28", so who knows.
For comparison, my OnePlus from 2024 has a 70mm f/2.6 with a 1/2.0" sensor. It takes very nice photos, but the newer vivo and Oppo are better.
OnePlus have good camera's on their flagship's but I hope they equally improve their budget offers as the camera quality on those isn't good. And also give or improve on telephoto lens for them too. Good zoom shouldn't be only available for 1000+ phones.