Related Clues:

• Element in water
• Chemical element
• The 'O' of H2O
• You can't live without it
• Space shuttle supply
• About 21% of air
• Product of star fusion
• 20 percent of the atmosphere
• Atmospheric gas
• Ambulance tankful
• What we breathe
• Breathing need
• Element in chlorophyll
• Astronaut's supply
• Element in laughing gas