If you mean molecular oxygen, O2, then no, there is no more than trace amounts on Mars (compared to the Earth which has an atmosphere composed of over 20% O2). Our atmosphere is mostly nitrogen (N2) followed by oxygen (O2) with smaller amounts of argon and carbon dioxide.
A ton is 2000 pounds (907.19 kg), so does it follow that a half-ton truck weighs 1000 pounds (453.5 kg)? Actually, the name doesn't refer to the weight of the truck. In fact, half-ton pickups weigh much more than that. The weight of a truck without cargo or passengers is called the curb weight. For example, many 2007 half-ton pickups have curb weights between 9000 (4082.3 kg) and 11000 (4989.5 kg) pounds [source: Edmunds].
In the United States and formerly Canada a ton is defined to be 2,000 pounds (907 kg). Where confusion is possible, the 2240 lb ton is called  long ton  and the 2000 lb ton  short ton ; the tonne is distinguished by its spelling, but usually pronounced the same as ton, hence the US term metric ton.