A green plant uses sunlight to convert CO2 and H2O to sugars and oxygen, it stores the sugars as food in starches and makes cellulose out of it to grow. But a plant lives by respiration which combines the sugar with oxygen to produce energy, CO2 and H2O. A static plant does not add oxygen to it's environment but a growing plant or a plant producing food crops does add oxygen to it's environment. Plants all consume O2 and produce CO2 at night as there is no sunlight, you only hope that it produces more O2 during the day. A dead plant releases it's carbon as CO2 as it biodegrades. A mature forest only produces oxygen because some of the dead plant material are sequestered by sedimentation. The full plant life cycle is carbon neutral and hence oxygen neutral if allowed to fully biodegrade. If your plant is an ornamental plant that is not growing in size and not producing food crops, it's probably not producing a lot of excess oxygen for you.

If it IS green, then definitely. Chlorophyll (the chemical that causes photosynthesis (CO2-oxygen transfer), and makes plant food) is what gives plants that green colour, thats why saplings are green all over, they need the extra coverage to transfer more CO2 (which in turn makes more "plant food"). Hope this was helpful :)

Is it an alive green plant? Then yes

YES, always!!