Which tree gives the most oxygen?

Fast growing trees like ash, poplar, willow etc produce most oxygen - because the amount of oxygen produced depends on the amount of carbon sequestered. Younger trees produce least, and old mature trees produce most per tree per year. 

According to it, a fast growing deciduous tree such as ash, cherry, elm, oak, poplar, sycamore, walnut, willow etc will sequester 2.7 pounds of carbon per year in its first year, increasing to 150.6 pounds (68.3 kilograms) per year at age 59. A fast growing conifer, such as some of the faster growing varieties of pine trees will sequester 1.4 pounds in its first year, increasing to 134.1 pounds (60.1 kilograms) per year age 59.

Deciduous trees also produce more oxygen for their leaves, but the leaves decompose and take that oxygen back from the atmosphere at the end of the year. So these calculations just take account of the net amount of oxygen produced per year.

Generally 50% of the biomass is carbon for dry wood.

 with some indication that 49% is appropriate for deciduous trees and 50% for conifers.

You can estimate the oxygen produced from the carbon sequestered . So, assuming one molecule of O2 is produced for each atom of carbon sequestered, then carbon has atomic weight 12 and oxygen, atomic weight 16, so that would be 32 grams of oxygen produced for every 12 grams of carbon sequestered. So a 59 year old deciduous tree would produce 68.3 *32/12 or 182 kilograms of oxygen per year.

Note, 70 to 80% of our oxygen though comes from algae and the sea. Also have plenty of oxygen in the atmosphere - even if somehow all photosynthetic life was destroyed, and there is no plausible natural or manmade disaster that could do that, it would take thousands of years before the oxygen levels went down, plenty of time for the vegetation to grow back. See my