applicationVariants.all { variant ->

    variant.outputs.each { out ->

        def oFile =out.outputFile  // outputFile causes failure

        //...
    }
}

This build error occurs because variant-specific tasks are no longer created during the configuration stage.
This results in the plugin not knowing all of its outputs up front, but it also means faster configuration times.
As an alternative, we will introduce new APIs to provide similar functionality.

API change in variant output 

// If you use each() to iterate through the variant objects,
// you need to start using all(). That‘s because each() iterates
// through only the objects that already exist during configuration time—
// but those object don‘t exist at configuration time with the new model.
// However, all() adapts to the new model by picking up object as they are
// added during execution.
android.applicationVariants.all { variant ->
    variant.outputs.all {
        outputFileName = "${variant.name}-${variant.versionName}.apk"
    }
}