Welcome to science buddies! This is a great, but really challenging idea for a science project. Earth's atmosphere is composed of about 20% oxygen, so to vary the concentration, it seems that you would need a completely sealed environment and compressed oxygen to use. However, I think it would be a good idea to do some background reading first and find out how other researchers have done this type of experiment in the past and find something that you could do.
For example, here is a paper on using dissolved oxygen to increase the growth of lettuce grown hydroponically. The researchers used an "oxygen enricher," to increase the oxygen concentration. Unfortunately, only the abstract is available, so it's not possible to tell exactly what an oxygen enricher.http://www.ifac-papersonline.net/Detailed/42816.html
Here is another paper and the entire text including the materials and methods section for a similar study using tomato plants. The tomato plants were grown hydroponically and the oxygen was increased by bubbling air through the nutrient solution. This sounds like a feasible experiment, except that you would need hydroponic supplies and some way of bubbling oxygen into the nutrient solution. Look through the paper and see if you can find out how the authors measured the oxygen concentration. http://link.springer.com/article/10.102 ... 13?LI=true
I recommend going to Google Scholar and doing a search for "effect of oxygen on plant growth," and see if you can find any other suitable experimental protocols. Also do a search for "how to measure oxygen," to find a good method.
Another idea. Since your project deadline is about a month away, and since this is a plant project, which will require time for the plants to grow, maybe you could use a rapidly growing water plant for your test subject.
Please post again to let me know what information you find, and if you have any specific questions on the information.