OneDrive is the free online storage from Microsoft , its for storing all your files and you can get to them from your iOS device, computer (PC or Mac) and any other devices you use.

And now with the latest update of OneDrive app for iOS, it supports Apple Watch too. It says:

‘If you have a new Apple Watch, you can enjoy your OneDrive photos right from your wrist!’

So Microsoft is ensuring that OneDrive will be there from the day Apple Watch will be seen on wrists of its user.
Seems the feature that would be available on Apple Watch will be to browse all the photos on OneDrive right from your wrist.
Anyway, one should appreciate Microsoft for providing the OneDrive update for Apple Watch right from the day one its made available.
You can get the OneDrive for iOS from iTunes . Can also check some more improvements in the new Version 5.3