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].
The short (net) ton equals to 2000 pounds and used in North America, the long ton (also known as weight or gross ton) is used in imperial system (UK and some of the other English speaking countries) and defined as 2240 pounds or 1016 kilograms and finally the metric ton (also known as tonne) equals to 1000 kilograms and used in metric system.