Oh dear, you forgot to buy more Choji. It's not the first thing you consider adding to your sword related shopping list is it? First off don't panic. I mean, the short of it is that you'll need to buy some at some point real soon, especially if you're planning on using your sword with any frequency, but there are a couple of things that you can do in the meantime in order to reduce the chance of damage to your blade.
Firstly, you should at the very least make sure that the blade is as close to bone dry as possible. You should have a powder ball and cloth to help with this. There can be no rust if there is no moisture.
Secondly, you'll need an alternative to Choji that will do in the interim. There are several options available, none perfect but you're in a pinch so; Mineral oil is available from the chemists. Razor oil is a little thin but you may have some already if you have beard trimmers. Bike oil is fairly neutral and does a good job. Light machine oil, again a little thin but it will work. Scraping the bottom of the barrel here but 3-in-1 oil will work.
Ones to avoid at all costs are Vegetable oils such as olive oil which will just turn nasty, yuk. Also, Never WD-40! Do not use this, it is not a protective oil.
As a final pointer, if you havent got a little plastic tub for your oiling cloth then get one. Putting your oil cloth in a little tub after youe used it will preserve the oil left on it for next time. You seldom need a lot of oil to protect your sword so this little tip will make the oil you do have go that little bit further and maybe next time you won't run out so soon.