The Occupational Injury and Illness Classification System (OIICS) was developed by the Bureau of Labor Statistics (BLS) to provide a standardized coding system for characterizing work-related injuries and illnesses. The OIICS has five subcomponent structures. Two structures are used to describe the characteristics of the injury or illness (i.e., "Nature" and "Part of Body Affected") and three structures are used to describe the incident circumstances (i.e., "Source of injury or illness", "Secondary Source", and "Event or Exposure").

Each structure in the classification system is comprised of rules of selection; a hierarchical coding scheme with up to four levels of detail represented by numeric codes with 1-4 digits; code descriptions; and alphabetical indices. At the highest level of the coding scheme are division titles (1 digit) followed by levels with successively more detailed descriptors: major groups (2 digit), groups (3 digit) and finally the fourth level which provides the most specificity for each component in the hierarchical coding structure (4 digit). Category “titles” occur at division, major group, and group levels and are indicated by a numeric code followed by an asterisk (e.g., for Source of injury or illness: 4* Parts and materials, 41* Building materials—solid elements, and 411* Bricks, blocks, structural stone). Titles serve as aggregation points and are not used to code specific cases. In the example sequence, an individual case would be assigned one of the 4-digit Source codes: 4110 Bricks, blocks, structural stone, unspecified; 4111 Bricks and pavers; 4112 Concrete blocks, cinder blocks; 4113 Structural stones or slabs, n.e.c.; or 4119 Bricks, blocks, structural stone, n.e.c. (n.e.c. = not elsewhere classified).

Two subclasses within each of the second through fourth levels are used to classify characteristics that can not be classified with more specificity (i.e., characteristics that are unspecified; e.g., 4110 Bricks, blocks, structural stone, unspecified) and characteristics that can not be specifically fit into one of the subclasses (i.e., characteristics that are not elsewhere classified; e.g., 4119 Bricks, blocks, structural stone, n.e.c.). When a group does not lend itself to further subclasses at the third or fourth levels the hierarchical scheme is truncated.

Individual worker injury and illness incidents are coded to two to four digits, depending upon the OIICS subcomponent and the amount of information available with which to classify the case. For analysis purposes, similar cases may be lumped at any of the four levels from division titles to the most detailed 4-digit classifications.

The Source and Secondary Source subcomponents utilize the same hierarchical coding scheme. However, the coding rules for the two structures differ slightly. Thus, there are four unique hierarchical coding schemes or coding trees within OIICS: Nature; Part of Body Affected; Source and Secondary Source; and Event or Exposure.