Apache OpenOffice Community Forum

I'm considering buying a 12.9-inch iPad Pro with a Magic Keyboard. Is OpenOffice compatible?

This sounds like a tablet and there have been a number of topics regarding OpenOffice for tablets.
To summarize, you'd probably be better off installing LibreOffice on the iPad

Is OpenOffice compatible?

At the time of writing, there is no version of AOO for iOS. LibreOffice has released Collabora Office which is available from the App Store.