Highly sensitive and wearable gas sensors are highly demanded on account of their widespread application in health and environmental monitoring. However, the sensing performance of conventional room-temperature gas sensors are prone to fluctuate or even fail under changing humidity, temperature or putting into water, thus severely limiting their practical applications. Herein, a porous elastomer-encapsulated hydrogel-based oxygen sensor is designed with impressive room-temperature sensing performance under various environmental conditions, including high precision (at ppm level), wide detection range (from 5 ppm to 90% O2), excellent repeatability, selectivity, long-term stability, breathability, and waterproofing. The hydrophobic elastomer film prevents the water molecules from intruding and evaporating to a great extent, making the device resistant to humidity interference and applicable to underwater operation. Benefiting from the intrinsic stretchability of both hydrogel and elastomer films and their strong interfacial bonding, the final device can be stretched up to 100% strain. By integrating the sensor with a wireless circuit module, a wearable/portable oxygen sensing system for real-time monitoring of health and environment is demonstrated, realizing timely and convenient assessment of respiratory rate, tissue oxygen, ambient oxygen, and dissolved oxygen.