The OnePlus 5T features two cameras at the back of the phone. The first camera is a Sony IMX398 Bayer sensor and the second one is a monochrome Sony IMX376K sensor. The good news is that both sensors feature a f/1.7 aperture lens. This helps to improve the low light performance especially considering the small pixels of both sensors (1.12-micron and 1.0-micron respectively.
For those of you who don't know, the f-number represents the opening of the aperture of the lens. The smaller the f-number is, the higher the amount of light that passes through it. This is why if you care about high-quality low-light photography, you should search for a phone with a camera that has a wide aperture.
The aperture is not the only thing that reflects on the low-light performance. I was quite disappointed to see that OnePlus has decided to go with such a high-resolution sensors which I have no doubt will have a negative impact on the low-light performance. That being said, it still great to see that there was done something to reduce the negative effect on the image quality due to the large pixels. I personally prefer having a phone with 8MP sensors, but nowadays numbers still speak to many people.
A larger aperture also affects the field of view. Generally speaking, you can achieve a more prominent Bokeh effect with a fast-aperture lens. The thing is that the sensors are so small, that the background blur effect is visible only in close-up shots. This, however, won't be an issue for many people as the phone can produce a very strong background blur effect due to the use of the dual cameras with the B&W sensor.
So I mark the F1.7 aperture lenses as an advantage for the OnePlus 5T because regardless of the other specs, it's still an advantage by itself and it will help promote better low-light performance in general.