Download and install OS X

iPads do not use OSX or macOS. iPads use iOS or more recently iPadOS. Apple computers use OSX or macOS.

Are you asking how to reinstall iOS onto the iPad?

Or are you asking how to install OSX or macOS onto the computer?

Please confirm and provide more information about the problem you are having with your computer and/or with the iPad?

Also, provide the exact model of the Mac and version of macOS being used. You can get this information by clicking the Apple menu and selecting "About This Mac".

Also, see this article for tips on asking questions on forums so that enough pertinent details are provided for contributors to understand the situation and the question.

Writing an effective Apple Support Communities question - Apple Community