macOS Compatibility of an application developed on mac os x

If the App is written in Carbon (and any additional supporting libraries are available for OS9) then the App will work on OS9 (and maybe later versions of 8). If you use Cocoa then it will definitely not work on OS9.

QT could mean 2 things here: QuickTime which should be fine for OS9 or Troll Tech's QT GUI/App toolkit. As there is no port of QT to OS9 that I am aware of then this would only run on OSX.

robbieduncan said:

If the App is written in Carbon (and any additional supporting libraries are available for OS9) then the App will work on OS9 (and maybe later versions of 8). If you use Cocoa then it will definitely not work on OS9.

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Just to clarify this, it has to be written in subset of Carbon, the subset supported by CarbonLib. It also has to be CFM (which requires you to get a copy of CodeWarrior because Xcode doesn't do CFM). And it is generally a pain in the arse that isn't worth it. Pretty much everything new requires 10.2+.

Rincewind42 said:

Just to clarify this, it has to be written in subset of Carbon, the subset supported by CarbonLib. It also has to be CFM (which requires you to get a copy of CodeWarrior because Xcode doesn't do CFM). And it is generally a pain in the arse that isn't worth it. Pretty much everything new requires 10.2+.

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Yes, depending on which libraries/OS features you want to use -- it may not work on older OS versions, even though it is a Carbon Application.

If you want it to work in OS 9, you'll probably be stuck with basically developing a new OS 9 Carbon application.