The botox injections in the forehead are very effective in smoothing the skin. This does change the way light reflects off, and can give a shiny impression. There is not any change in the actual secretion of oils from the skin, as far as I have been able to research.
What you are experiencing after Botox has been injected into your forhead is the relaxation of the forhead muscle and a smoothing of the overlying skin. I have noticed over time that people who are consistent Botox users do notice skin across the forehead being smoother, flatter and more luminous. That can be confused with oiliness but it is not.