Today 1 atm (101.3 kPa, 14.7 psi) with sea level composition is most commonly used on spacecraft to avoid fire hazards like in Apollo 1 (not that it would have helped the Apollo 1 crew as they couldn't exit the capsule, it would only have postponed their deaths I guess). But the Apollo spacecraft used 5 psi (34 kPa) onboard pressure. Space Shuttle spacesuits use 4.3 psi (29.6 kPa). U2 and Mercury spacesuits use(d) 3.7 psi (25 kPa) psi of pure oxygen. That's still more oxygen than at sea level.
If spacecraft, -stations and -suits were pressurised to 2.5 psi (17 kPa) psi of pure oxygen, wouldn't this both make them lighter and better prevent potential hazardous fires? It's an oxygen level similar to 4,500 ft (1,370 m) above sea level, so astronauts should be fine, shouldn't they?