OnePlus has been fighting an oil painting effect in its cameras for a while now the OnePlus 5 and 5T have it, the OnePlus 6 has it too. So heres a bit of good news: fixes are on the way. The 5/5T fix is already being tested through the Open Beta program and should roll out soon (it seems that the update is already rolling out in China).
OnePlus confirms it is working on a fix via email
The new model, OnePlus 6, will get a fix as well, which will roll out with the next software update (after passing through the beta stage, of course).
It seems that the issue is caused by the HDR mode and turning it off reduces (or even eliminates) the effect. The oil painting effect looks like this:
OnePlus “oil painting” effect”: HDR on HDR off HQ mode
Source | Via