OnePlus 6 is a lot costlier than Nokia 7 Plus. The price difference is around 10,000 bucks, which is huge and so are the differences in features.
OnePlus 6 has a bigger display, faster processor, higher resolution camera and dash charge support. If the concern is not about the money, OnePlus 6 is the best thing one can have at such a price range. Else, Nokia 7 Plus is also a great phone.
You can see the detailed comparison here: Nokia 7 Plus vs OnePlus 6
Nokia 8 Sirocco is almost 10,000 bucks costlier than Nokia 7 Plus. However, the difference is features in not that much to justify the extra cost. Nokia 8 Sirocco has a QHD Display, a bit faster processor and 100% more storage. While on the other hand, Nokia 7 Plus has a higher resolution front camera and bigger battery capacity.
I would personally recommend going with Nokia 7 Plus, it has got all the necessary features that you might require.
You can see detailed specs here: Nokia 7 Plus vs Nokia 8 Sirocco
Jobanpreet Singh Batth