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.