No, indeed! As you know, plants put out oxygen. When our Earth was a couple of billion years old, water filled the low spots and became the oceans. (Hugely different in location and shape than now).
In those oceans life was created. One of the earliest life-forms was a blue-green algae. Blue-green algae is very much like a plant, and it even put out oxygen. So it was billions or trillions of these tiny algae that produced the first oxygen in Earth's atmosphere.
The ocean today still produces most of the oxygen on Earth.