A high-fat meal can improve bioavailability of certain Opioids. I'm not entirely positive as to what extent lipid content of the stomach effects Oxycodone's metabolism, but as a general rule of thumb, having a meal in the stomach, preferably high in fat, cannot hurt and will probably help. The only difference is that the onset of effect is going to be slightly prolonged, which could be perceived as a weaker hit from the drug.
There's really no need to worry about having or not having food in your stomach when regarding taking Opioids. For reasons concerning nauseau and/or bioavailability though, having a meal in the stomach is probably a good thing.