Here is something you can do to see that a plant does make oxygen.

 You will need Mom and Dad's help on this one.

Buy 2 water plants like you use in your aquarium.  These are available at a fish store.  You need 2 large jars with big openings, 2 funnels and toothpicks.

Put a plant in each jar and fill with water, then turn a funnel upside down over each plant.

Now take the test tubes and hold your finger over it and turn it upside down over the funnel end and lower into the water.  Now move your finger and place the test tube over the funnel end.  You want to do this with both plants.

Now put one jar outside or where there is good light.  Put the other one in total darkness like a cabinet or closet.  At the end of the day get each jar carefully and put your hand under the test tube and put your thumb over the test tube end and take it out off the water.

Keep your thumb over the test tube opening. Now light a toothpick (or have Mom of Dad do this) so that it flames. Blow out the flame and immediately (still glowing) put it in the test tube. Do the same thing with the test tube from the dark jar.

What happened.  When you put the toothpick in the test tube fromt he plant in the light it should have relight and the other toothpick did nothing.  That is because there was oxygen in the test tube from the plant in the sun and none in the plant in the dark.  Because of the sun the plant produced photosynthesis.