Oxygen builder is forever tied in with other names like Elementor, Beaver Builder, Divi, etc. but it really isn't the same thing. Oxygen feels a lot more like coding with a GUI than using a conventional page builder. The amount of control you have over everything is so much better than with any other builder. Other builders give you a lot of drag and drop components to create the content you want. These components are usually filled with lots of baked-in features and options, but it can be a bit tricky to change them in a way that the builder developers didn't intend. Oxygen is a bit different in that it gives you all the tools you need to make that component instead. Using the tools they give you, you can build it exactly the way you want it. This allows you to make the perfect design rather than one that's just close enough.
How It Works
Oxygen gives you some very basic drag and drop components such as sections, divs, columns, text, images, etc. They also provide a few of what they call "helpers". These are a little bit more like what you see in a lot of page builders, but they are less feature-filled and much more flexible. These include things like a header-builder, slider, tabs, and a repeater.
The way you create your pages (and templates) is much more similar to the way that you would do so manually with code. Rather than inserting made-up builder lingo modules, we use divs. Just like we would if we were coding from scratch, we start by building a sort of framework out of divs then add our content inside. From there you can adjust every little thing about each element easily from the interface. If there's something less typical that you want to do, you can easily add custom CSS to the element as well.
One of the most unique things about Oxygen is that it actually disables and replaces the default WordPress theme system entirely. Oxygen essentially is your theme. In Oxygen, you can create templates for everything that you'd normally make a template PHP file for in a theme such as a header/footer, archives, singles, special page types, etc. This aspect of Oxygen is similar to what Beaver Builder's Beaver Themer plugin adds.
This video is a showcase for a plugin called OxyNinja Core Framework which is basically a CSS framework you can import into Oxygen, but I think it's a great example of what a good workflow in Oxygen looks like if you are using CSS classes right. With or without OxyNinja, this is what building a page should look like once you've established some of your common CSS utility classes and such.
Pros and Cons
Stylish and intuitive interface
No made-up builder lingo
Uses real terminology instead (ex: div, class)
The way things are built using divs and classes is much closer to a real developer workflow than other builders
Steep learning curve for people unfamiliar with more traditional web development practices
Oxygen's current price model is, in my opinion, an insanely good value. I expect they will change to a yearly subscription model at some point when they own a larger percentage of the page builder market share, but for right now they only offer lifetime licenses and those lifetime licenses are a similar price to the yearly cost for other builders. $99 gets you a lifetime license, $149 gets you a lifetime license plus including the WooCommerce Integration, and $169 gets you a lifetime license plus the Gutenberg Block Builder and all future additions.
Is Oxygen Builder For Me?
If you don't really know how HTML and CSS work you might have some trouble getting started with Oxygen Builder. With that said, I think with the right attitude and effort anyone can learn to build with Oxygen. There will be a steep learning curve, but you will become a much better and more versatile developer/designer for it.
If you have no desire to learn the more intricate parts of web development/design and just want to click and drag to make decent designs I don't recommend Oxygen for you. For this group of people, I recommend something like Beaver Builder(Disclaimer: THIS IS AN AFFILIATE LINK) instead. Beaver Builder offers decent performance in terms of page speed, tons of features, and an easy to use drag and drop interface.
I love Oxygen Builder and I plan to use it for a long time. I think it's the perfect middle-ground between coding from scratch and using a typical page builder. Using Oxygen doesn't make me feel limited in what I can do at all. Workflows in Oxygen closely mirror that of a coding from scratch approach and promotes doing things the right way, rather than throwing something together with a bunch of drag and drop modules and bloating your site with random garbage you don't need. Creating a site takes a bit longer in Oxygen than with other builders, but that's mostly because, if you're anything like me, you end up spending an inordinate amount of time nitpicking every little thing until it's perfect, which other builders just don't really let you do.