Download Onshape standalone app (release notes).
“Onshape is the first and only full-cloud 3D CAD system that lets everyone on a design team simultaneously work together using a web browser, phone or tablet. CAD Anywhere, Anytime, On Any Device.”
So, there is no official desktop application since you can use Onshape on a desktop computer using web browser.
But using web browser in this case is not always convenient — you lost all standard ways to open app or to switch to it. What if you want to easily switch to Onshape using cmd+tab? And not to lose Onshape among other web pages?
Please note – it is unofficial application.
You don't need to worry about updates, once installed, application will be updated automatically (in the same way as Google Chrome).
Nothing special. Just download, double click to mount it and drag the application icon to the /Applications folder.
After the app is dragged to your Applications folder, you can run it normally — from the Finder, Launchpad, Spotlight, dock, or anything else. The first time you open a downloaded application, you’ll see a warning. Agree to open the file and you won’t see the warning again. See short or long (“DMG Files" section”) guide.
Download, open the folder that contains the downloaded file, double click to open it, application will be automatically installed and launched. Desktop shortcut will be also created.
You can ask questions, request improvements and file issues in the tracker.