Unfortunately, the OnePlus Open doesn't have a microSD card slot or any other way to expand the internal storage offered when you buy the device. Also, the OnePlus Open only comes in one storage size — 512GB — which means you can't even opt for more storage at the time of purchase...
There are some exceptions for high-end phones that still have microSD card slots. Sony's phones are often built for creators, so having a slot is important, and you'll find one on phones like the new Xperia 1 IV. Even the Galaxy S10+ from years ago had up to a whopping 1TB of st...