Limited research. It is always difficult to find organized research on natural suplements but oxyelite contains a stimulant and there is always a risk of serotonin syndrome when combined with ssri medications like citalopram.
No studies. No studies have been done with people using both oxyelite pro and citalopram. Oxyelite pro contains several herbs purported to increase fat burning, plus caffeine. One of the herbs (bacopa monniera) can cause diarrhea, nausea, and stomach cramps -- just as citalopram and other ssri's can do. Because of this and possible increased anxiety with oxyelite, I wouldn't recommend using them together.