Just put the Betta in a large jar or a vase for the week. This way you do not have to worry about it. Betta will do fine for a week in a smaller container. It will be the Taj Mahal to him compared to the tiny cups that they live in at the stores.
I put my Betta in a tank with two garamis and a butterfly fish and he vaporized overnight the first night. Not a trace anywhere. I personally wouldn't and just do what other say. Put him in a vase for a week.
Definetly either hold off getting the betta until you can move the oscar into a bigger tank, or put the betta in a vase or small 1 or 2 gallon aquarium. You can get a nice 1 gallon tank for $7, and it could be a temporary home for your betta until the 10 gallon is vacant.
I guess they don't need it (I think I've read they tolerate a range of temps. better than some other fish), but it is really best, plus it will help keep the temp. constant, which is very important