Like most things in science, there isn't a specific answer.  However, introducing oxygen into the lattice structure reduces the density of the material and there's a crude link (Gladstone-Dale relationship) between density and optical properties (refractive index). For a particular example of these relationships see slides 14 and 22 in:

August 11th 2011 Optical properties of the three forms of titanium dioxide  http://tinyurl.com/pbgf9hj

In all cases as the wavelength reduces both n and k in RI = n - i.k tend to increase.

Bunch of webinars in this area.  Start at:

Jan 24th, 2006.  Optical Properties: the Gladstone-Dale approach

http://www.brainshark.com/brainshark/vu/view.asp?pi=141109&text=M021507

Jun 27th, 2006 Optical Properties - use and abuse  http://tinyurl.com/jo2u6p9

Sept 21st, 2006 Optical Properties - Becke Lines, Immersion Fluids

https://www.brainshark.com/malvern/vu?pi=577723565&text=M021507&r3f1=&fb=1

Doping oxygen  has a significant effect on the absorption characteristics of wide band gap semiconductors. The insertion of oxygen into the matrix of semiconductors   forms abundant doping energy levels as well as creating a charge compensating  vacancies in the lattice points of metal oxides.  Existence of oxygen ( intrinsic point defects) into lattice semiconductor structure acting as nonradiative recombination centers. Depending on the type of semiconductor, oxygen   play a dominant role in the conduction mechanism which has an effect on optical properties of semiconductors.

Plz refer to:

1-Electronic and optical properties of the doped TiO2 system, Journal of Semiconductors, Vol. 31(7),  2010.

2-Role of oxygen vacancies in V-doped ZnO diluted magnetic semiconductors Journal of Materials Science Materials in Electronics 26(4),2015.

3- Fundamentals of zinc oxide as a semiconductor, Rep. Prog. Phys.72,2009.