In the process of photosynthesis, several products are formed within the chloroplasts of plant cells. Photosynthesis involves the conversion of light energy into chemical energy, resulting in the production of glucose and oxygen as its primary products. Let's evaluate the options given:

Option A: ADP

ADP (Adenosine Diphosphate) is not a product of photosynthesis. Instead, it is involved in the cellular respiration process and is converted to ATP (Adenosine Triphosphate) during the light-dependent reactions of photosynthesis.

Option B: NADPH

NADPH (Nicotinamide Adenine Dinucleotide Phosphate) is a product of the light-dependent reactions of photosynthesis, but it remains within the chloroplast to be used in the Calvin cycle for the synthesis of glucose. Hence, it does not diffuse out of the chloroplast.

Option C: O$_2$

Oxygen (O$_2$) is a byproduct of the light-dependent reactions in the thylakoid membranes of the chloroplasts. This oxygen is released into the atmosphere and hence, diffuses out of the chloroplast.

Option D: ATP

ATP (Adenosine Triphosphate) is also produced during the light-dependent reactions of photosynthesis. However, like NADPH, ATP is utilized within the chloroplast for the Calvin cycle and does not diffuse out.

Therefore, the correct answer is:

Option C: O$_2$