In simplest terms, ODC or Over Dimensional Cargo is a cargo that protrudes outside the loading deck of the vehicle transporting the cargo, cargo that extends beyond the normal deck of the cargo (may it be 20' or 40' or more). The over-dimensional cargo gets transported in flat racks and open tops. The freight rate borsch cargo differs from the regular sea freight rate and is calculated based on the cargo's dimension being transported.
For example: If a truck with a loading platform length of 20 feet is loaded with cargo like TMT bars of length 22 feet, then the TMT bars qualify as Over-Dimension Cargo. If the same TMT bars were loaded on a vehicle with a platform length of 22+ feet, it would have been classified as Normal Cargo rather than ODC.
The government imposes fine on the carriage of ODC, hence incentivizing the industry to refrain from taking unnecessary risks and using larger vehicles to transport these loads.