The major and top source of oxygen in the atmosphere is

✏️Photosynthesis by oceanic plants:
-Oceanic plants, particularly phytoplankton, are the major contributors to the production of oxygen in the Earth's atmosphere.
-Phytoplankton are microscopic organisms that live in aquatic environments and perform photosynthesis, converting carbon dioxide and sunlight into oxygen and glucose.
-It is estimated that these oceanic plants contribute to more than half of the Earth's atmospheric oxygen.

🔴Additional Information:

✏️Photosynthesis by forest plants:
-While forest plants also contribute to oxygen production through photosynthesis, their contribution is significantly less compared to oceanic plants.
-Forests are crucial for maintaining local ecosystems and carbon sequestration, but they do not match the global scale of oxygen production by oceanic plants.

✏️Photosynthesis by grassland ecosystem:
-Grasslands also play a role in photosynthesis and oxygen production, but their contribution is minor in comparison to both oceanic and forest plants.
-Grasslands are important for biodiversity and soil health but are not the primary source of atmospheric oxygen.