Problem using "controls" command

[OnkelHo27]

Hello everybody,
I have a slide with many labels and named them "o_1", "o_2" and so on. I would like to access them using a loop. I found the "Controls" command as a potential useful tool. However, it does not work. I do not get any error messages, but the code is not executed. A simple example:
This works:

If o_1.BackColor = 0 Then
    MsgBox "Hello World!"
End If

This does not:

If Controls("o_" & 1).BackColor = 0 Then
    MsgBox "Hello World!"
End If

Any ideas why this is not working?

Thanks a lot!

Felix