First, if you plan to use the same tank for flight and storage, that's not a good approach, because the tank to last without leaking, corroding, or being at risk of damage, needs to be quite thick. Use a dedicated oxygen storage tank (like a gas welding tank), and a small 'flight' tank for the rocket, filled right before launch. It doesn't need to be up to quite high standards, as both the amount and duration of exposure to oxygen won't cause a significant risk.
You should be able to adapt a tanks used for Airsoft gas pistols - there's a wide range of sizes, they are refillable, can hold a respectable pressure, and you can get all the "plumbing" as spare parts for ASG pistols.
Although, if I were you, I'd go for nitrous oxide. The cartridges "whippets", while not refillable, are cheap enough to treat them as disposables, safe, light, and you can get all the "plumbing" from a whipped cream dispenser, for which these cartridges are made. Never mind they hold 8 grams of nitrous oxide for 22g of dry mass. You'll never get this weight ratio out of $O_2$ unless you go with liquid oxygen.