How do I reference a method in an app created with CreateObject()?

For example, if I have

oApp = CreateObject( "MyApp")

for which MyApp contains a method DoThis(), I can, within MyApp, successfully call this.DoThis(), but if later I have an object in a form opened by MyApp, I cannot figure out how to call that method.

Is it possible, or do I have to keep DoThis() in a proc file?

Thanks!
David Chan