The O2 sensor shouldn't keep the car from starting and it's pretty
unlikely that it needs replacing unless the car's got over 100K miles on
it. It sounds like the guy saw you coming and is looking to make a big
ticket repair. I'm also skeptical about the timing belt. They only way
it can be "loose" is if the tensioner is failing or has failed. It's not
something that you can adjust. Again, I think you're being set up. Take
the car elsewhere and get another opinion.

I have difficulty imagining how the Oxygen Sensor would keep the vehicle
from starting.  You may well have a faulty oxygen sensor, however.

If your vehicle has enough miles on it that it's out of warranty, then you
should replace the timing belt if it has never been done, but I also doubt
this is your problem, although it is remotely possible.  I similarly doubt
that it is loose.  I don't think I've ever seen this on an Accent unless
the tensioner pulley went bad.

Check with the dealer to be sure you understand them correctly that all of
these items are necessary for the vehicle to start.  Not all service
advisors make clear the difference between repairs necessary to correct
the condition for which you brought the car to them and those repairs
recommended in addition for a variety of reasons.  Technicians also many
times do not give the advisor enough information to know what is
necessary, what is additional, and why the repairs are recommended.

If the dealer continues to claim that the oxygen sensor is part of the
no-start issue, then you should probably go elsewhere.