OnePlus is good enough to provide a beta channel for its phones, so you can stay on the bleeding edge. You’ll also probably encounter more bugs, but such is life. There are new Oreo-based Open Betas live today for OnePlus’ recent phones. They include some functional updates as well as security patches.
All devices appear to share the same changelog, and here it is.
Now, we don’t know what “Hidded Space” is, but presumably OP meant “Hidden Space.” That’s not really clearer, though. You’ll have to install the beta to find out. You also get a new toobox card in the shelf and editing of dynamic icons on the launcher. OnePlus’ switching tool is getting some optimizations as well. The OnePlus 6 is coming soon, so that’s good timing.
The beta version number varies by device, but the May update is available for the OnePlus 3, OnePlus 3T, OnePlus 5, and OnePlus 5T. OP has full flashing instructions on the download page.