I began to notice that sometimes when starting a game in a helmet, the horizon line displayed in the helmet is not parallel to the actual horizon line of the surrounding room. That is, even with the settings of the protective zone, the screen in the settings is located parallel to the external horizon line, but when you load the game - in games, the horizon line is slightly tilted (by a degree and a half or two) to the left.
I found the reason by searching the Internet.
It turns out that the company Oculus had a program IMU Calibration Tool - a tool for calibrating Oculus helmets, in which you can correct various errors, including adjusting or removing the slope of the horizon. I found this program thanks to these links, which describe the principle of operation of the program (especially interesting is the setting and calibration of the second link) :
forums.oculusvr.com
www.vrheads.com
However, all references to the tool itself have disappeared. And Oculus itself has removed this tool from its website. But nothing disappears completely on the Internet. I found a link where you can download this tool:
Question to the owners of the Oculus helmet – has anyone used this tool to solve the problem with image tilt?
Please excuse my English.
I found the reason by searching the Internet.
It turns out that the company Oculus had a program IMU Calibration Tool - a tool for calibrating Oculus helmets, in which you can correct various errors, including adjusting or removing the slope of the horizon. I found this program thanks to these links, which describe the principle of operation of the program (especially interesting is the setting and calibration of the second link) :

Is your Rift's view tilted / slanted? Here's how you fix it
I am writing this thread in response to my experience with Oculus support. Oculus support has an extremely quick first line support, however they did not have the tools to deal with this issue, and the second tier seem to go through this dance of "please provide pictures of the issue", which...
Android Central - News, Reviews, Deals & Help on all Android devices
Your buyer’s guide for the best Android phones, deals, news, and reviews! Android Central covers smartphones, watches, tablets, ChromeOS devices, Google Home, and all compatible peripherals in this smart world we're living in. Read all this and more on Android Central.

However, all references to the tool itself have disappeared. And Oculus itself has removed this tool from its website. But nothing disappears completely on the Internet. I found a link where you can download this tool:
Question to the owners of the Oculus helmet – has anyone used this tool to solve the problem with image tilt?
Please excuse my English.