I've had a good experience with the official OnePlus Screen Protector
I guess it depends on how the installation goes. Proper application of the screen protector is more important than just the build quality of the protector, especially when the screen curves at the edges. Even a half mm shift on one side is likely to throw off the whole fit and lead to a bad experience, often translating into expensive disappointment and frustration with the product.
If you look at the packing an finish of the OnePlus TGSP, you'll realize that it wasn't thrown together carelessly, it was crafted with a lot of precision, by the people who probably understand the dimensions of the OP6 better than anyone else. Even I was very skeptical in paying substantially more for the official TGSP from OnePlus but went ahead and ordered it anyway.
Upon receipt of the item, I followed their instruction video exactly, using the case as a guide for application - and ended up with an amazing fit. No bubbles, no mismatch, no edge sticking off and no 3D or halo effect. It's just like the naked screen - had it installed for 10 days now and it's held up beautifully during everyday use. I haven't dropped it or banged it against anything yet, so can't comment on impact resistance (glass isn't built for that anyway, and I can live with that), but no blemishes so far. And I'm using a skin on the back, so at least I'm a 100% scratch proof now (just not shatter-proof).
Of course everyone's mileage may vary depending on the application. I might just as well have ended up with a botched application and vented my disappointment with the same product here. Maybe I was lucky, or careful, or both. I guess what I'm trying to say is that if applied properly, the OEM screen protector from OnePlus is very, very good, and maybe doesn't deserve the bad reputation it's been getting here.
Just wanted to share my experience, and this is not the universal truth, other users may have a different story to tell - if your experience has been different, please do share here.
I'm attaching some post-application pics for reference.