Have you concidered that it won't be pure oxygen since the tank (or whatever) which you use for capturing the oxygen molecules aren't empty before the experiment starts.
You can't vacumepump it out while it's in the water since that will only cause the water to boil which will fill up the "empty space".
Hehehe, but the tank(or whatever) I'm capturing the oxygen is turned upside down, and it is full of water at the beginning of the experiment. Its mouth is in water, so even like that, the water will stay inside and the oxygen molecules will slowly push the water away as they increase in number.
Edit: Oh and I'm not an expert, but, I don't think you will do any good with a simple 12V battery either. H2O is kept together with hydrogen bondings, and they don't give up very easily.
You can do it even with a 4,5 v battery, but I actualy use a PC power supply for this.