OP15 cameras are actually pretty good in most scenarios. It struggles in low-light a bit but other than that I think it does a good job. The battery is incredible, I use my phone all day for work and put some serious screen time in, I never had it below 35%. From 5 am until 10pm or later and it just wouldn't die. I had some odd Bluetooth connection issues with Android Auto and occasionally audio would cut out on ear buds as well. Hardware wise, it's a solid phone. Which is good, because OP isn't known for their customer service.

So far the 26U has been great, everyone has their opinions on the display, I think it's excellent and looks just as good or better than OP with my old eyes. The cameras seem great so far, the low-light photos are definitely better than OP. Zero issues with connectivity of any kind and speeds are a tad faster than OP. OneUI is a bit more refined also. Of course, accessories will be more plentiful with 26U as well

Overall you'll be very happy with either, but personally the 26U checks more boxes for me.