A little over a week after the initial Nougat-based OxygenOS 4.0 release, OnePlus has released a small update for the OnePlus 3 and OnePlus 3T. The new OxygenOS 4.0.1 build is rolling out to both devices currently and is primarily focused on addressing bugs and stability issues with the OS on these devices. As OnePlus does, the update will be released to a small number of users first then roll out to the larger population as confirmation all is well with the update comes into them.
As for what is addressed, probably the biggest news is the updated APN settings. After the initial OxygenOS 4.0 update, many 3 and 3T owners had network connectivity issues with their carrier. This update should resolve those issues. However, in the posting announcing the update, OnePlus is saying that if you still have issues, to manually adjust the APN settings or, worst case, report it to the product team at OnePlus with your phone model and carrier involved so they can help identify the issue and resolve it.