This is very true. The martian gravity isnt strong enough to stop oxygen from leaking away into space, but as you say, this process will take millions of years. There is a lot of oxygen on Mars alaready, the problem is that it is all bound up in H20 and CO2. The oxygen in CO2 can be released by photosynthesis but this too would take 100s if not 1000s of years to create an Earth like atmosphere (maybe even millions).
Technology would have to provide oxygen replenishing in the short term. Hydrolysis of water perhaps in huge solar power stations, or sliding a chunk of ice comet through the martian atmosphere. Either way though, a sustainable ecosystem would need to be established to maintain the oxygen level, and aside from transplanting entire rain forests from the Earth (we cant even mainatin our own level of flora anyway), that process will still take a very long time, probably not in our lifetimes