In food biotechnology, Pleurotus ostreatus is of great interest as a source of natural antioxidants and angiotensin-converting enzyme (ACE) inhibitors. However, research in this area has not yet been completed. The effect of various drying methods on the structural properties and the rehydration capacity of mushrooms was investigated in this paper. The content of secondary metabolites, the peptide profile, and the antioxidative effect and ACE inhibitory activity of dry mushrooms were investigated in vitro, simulating the process of gastrointestinal digestion. X-ray microtomography has confirmed that structure of lyophilic and sun-dried mushrooms is dominated by open pores, and in mushrooms dried with hot air and microwave, closed pores. Experiments have shown that the conditions of freeze drying and sun drying of Pleurotus ostreatus provide a higher rehydration capacity of dried mushrooms. The maximum activity of radical absorption of the oyster mushroom after microwave drying was observed. The iron restoring capacity of the mushrooms is maximally maintained with microwave drying and hot-air drying. The properties of the antioxidant product with an emphasis on the high activity of inhibiting lipid oxidation of the mushroom maximized after drying in the sun. Mushrooms dried lyophilically and in the sun showed the highest ACE inhibitory activity.