The controltiptext property works nicely on form controls, but is not available for OLEObjects on the worksheet. In that case, you can program a workaround using worksheet events, something like:
Private Sub CommandButton1_MouseMove(ByVal Button As Integer, ByVal Shift As Integer, ByVal X As Single, ByVal Y As Single)
ActiveSheet.Shapes("ToolTip"

.Visible = True
End Sub
Private Sub surround_MouseMove(ByVal Button As Integer, ByVal Shift As Integer, ByVal X As Single, ByVal Y As Single)
ActiveSheet.Shapes("ToolTip"

.Visible = False
End Sub
Here, surround is a rectangle (an image control, in this particular case) with background color and line color set to "window background" that encloses the button control. The ToolTip object is a TextBox shape created using the drawing toolbar, formatted however you like.
Rob
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