OnePlus' OxygenOS is an interesting beast. Insert obvious joke about it being "a breath of fresh air" here, given the name. While it looks like stock Android, for the most part, it adds quite a lot of functionality on top of Google's vision. There's a lot of that going on in various bits and pieces of the UI, and OnePlus seems to generally be erring on the side of presenting you with more choice. As that was a core tenet of Android's marketing message, it seems like OnePlus is basically out-Googling Google in this respect, especially now that the search giant is taking a more Apple-y approach to software design, removing certain options so as to not confuse normal users.
OnePlus' focus, on the other hand, has always been on the enthusiasts. And these are still served well by the latest iteration of OxygenOS. The software is, in a manner of speaking, the best of both worlds. You get the stock look - which means no unnecessary UI changes just for the sake of being different, no bells and whistles that are there only to make this particular skin stand out from what you'd see on a Pixel. At the same time, you also get a lot more customization at your fingertips, something that purveyors of custom ROMs have always appreciated.
It's an interesting mix, and it works. OnePlus isn't messing with any core functionality in Android, it's just adding what are usually neatly thought through options on top. It's not changing the general UI paradigm but is improving it in subtle ways. All of this doesn't appear to come with any performance penalty whatsoever, which is great considering that other UI overlays do add an overhead from that point of view.
There are still things that could be improved, changed, or altogether scrapped. Since it's got such a stock-looking vibe, OxygenOS would do very well to add the Google Feed (or whatever it's called this week) to the left of the leftmost home screen. OnePlus' Shelf is there now, and the good news is you can disable it. The things it contains can easily be found elsewhere - like recent contacts or the memo writing function. Others, like the recent apps section, could have just been added to the top row of the app drawer emulating the Pixel launcher.
As for membership cards, sure, that's a nice touch, but there are plenty of apps in the Play Store that have that functionality, including Google Pay. And if you use that for contactless payments, why not store your loyalty cards in it as well? Not to mention that if you ever get a phone that isn't made by OnePlus, you'll appreciate the card syncing ability third-party apps provide. When it comes to the Dashboard section, all of that information is accessible through Settings as well, so redundancy is pretty much the Shelf's middle name.
OnePlus also packs gesture-based navigation into the latest stable release of OxygenOS, but unsurprisingly this isn't the same as what Google's settled on for Android 9 Pie. In Pie-based OxygenOS releases you'll get that option alongside OnePlus', yet we can't help to think that the state of gestures in Android is currently pretty messy. Every manufacturer has its own interpretation, and that creates somewhat of a learning curve when you switch devices.
OnePlus goes with swiping up from the middle of the bottom of the screen to go Home, doing the same but pausing for the Recent apps menu, and swiping up from the left or right part of the bottom in order to go back. The system works well, and you'll get used to it in a few days tops, but we're sorely missing an ability to quickly switch to the previously used app.
If you decide to use the legacy three-button software navigation, this is possible if you double-tap on the Recents key. In Google's version of gesture navigation, this is accomplished by dragging the pill icon to the right. Perhaps in future OxygenOS iterations, OnePlus can focus more on this aspect, as it considerably reduces the friction in multitasking.
Our last note about gestures concerns Google Assistant, and the fact that if you opt to get rid of the traditional three-button navigation bar the only way to invoke it is by saying "OK, Google" or "Hey, Google". We're fine with that in private settings, but it could get weird in public.
The OnePlus Switch app is there to help you transfer your data over from your old phone. It failed to connect to the Pixel we tried first but then it transferred all the data from a Galaxy S9+, which we tried next. Unfortunately, no files were transferred - such as photos and videos - but outside of that, after the transfer, the OnePlus phone was usable straight away as it had all accounts.
The OnePlus Gallery app exists, though we're not sure why, in a world where Google Photos is a thing that can do all it does and more - don't forget the free unlimited cloud storage. The same observation goes for the Notes app - wouldn't it have been easier just to include Google Keep? So, unfortunately, OnePlus is also afflicted by the app duplication disease, although it's on an entirely different scale than the likes of Samsung.
There's a built-in voice recorder app, but this doesn't seem to be able to record calls with this latest version of OxygenOS. Sadly, Android Pie has also apparently made call recording much more complicated (if not outright impossible), so this feature will likely be phased out on all current and upcoming phones.
In Settings, OnePlus adds a whole Customization section, and hides some less used options behind the Advanced moniker. That's where you can find nice additions such as a scheduled power on/off, and a toggle for Pocket mode (which prevents accidental screen touches). You can customize the alert slider's behavior, namely what happens in all three of its positions - Silent, Vibration, and Ring. You can not, however, change what each position is - the lowest is Ring, the middle is Vibrate, and the top one is Silent no matter what.
If you use the classic navigation bar, you can also customize what long pressing and double tapping on each button do. Double-tapping the screen to wake is an option, and you can draw various letters on the display to accomplish specific tasks, or flip the phone to mute incoming calls and even take a screenshot by swiping with three fingers on the panel.
While the OnePlus 6 doesn't have an Always On Display function per se, you can get it to show you such a screen every time you lift it up. Additionally, it will show you every new notification you receive.
OnePlus historically has been among the fastest to update its older handsets to new versions of Android, but that doesn't mean there isn't any room for improvement. It still takes months for a stable release of the latest tasty treat to make it to a OnePlus handset, unfortunately.
If you like living on the bleeding edge, you can part take in the Open Beta program and test these while they're still in development and bug-prone, but we wish that swifter updates would become the norm. You probably shouldn't expect Android Pie to make it to the OnePlus 6 earlier than December or January. We're basing this assumption on how things have gone in the past but we'd be very happy to be proven wrong.
When it comes to security updates, things aren't great. Our OnePlus 6 is still on the July 2018 security patch level at the time this review's been published. Not one, but two newer monthly security updates have been outed by Google in the meantime. So while you do get updates from time to time, security fixes that actually arrive monthly aren't yet a thing for this model. Maybe in the future.