http://www.nytimes.com/1993/01/05/science/the-environment-ox... says "Project leaders, expecting the decline to continue, have made arrangements to pump tons of pure oxygen into the 3.15 acres of glass domes. ... "If we decided to raise the level by 2 percent, that would be 100,000 cubic feet of gaseous oxygen,""
A short ton (907.2 kg) of oxygen is 635 cubic meters or 24160 cubic feet as a gas and 794.5 liters as a liquid ( http://www.uigi.com/o2_conv.html ) so they would have needed 5 tons to raise the O2 levels by 2%. For what it's worth, the concrete depleted the O2 levels from 21% to 15%, so about 30 tons would have been need to fully restore the O2 levels.
800,000 mL/ton means that it was not the case that it needed "a few millilitres of liquid oxygen injected every 6 months or so." More like a few thousand liters every 6 months.