An OMB Control No or sometimes just OMB Number is a document reference number created and managed by the United States Office of Management and Budget. OMB Control numbers help to manage the lifecycle of documents circulating through the various federal government agencies. In most circumstances, OMB Control Numbers are valid for 3 years.
If a document is expired, you can often lookup an OMB control number to get the latest revision of the document. Search you OMB number at OMB.report.
The first four digits of an OMB control number identify to which Agency the collection belongs. The last four identify the specific collection belonging to that agency. The complete list of OMB Control Number prefixes and the associated agency may be found here.