Object access method is an access method under z/OS which is designed for the storage of large numbers of large files, such as images. It has a number of distinguishing features, e.g. compared to VSAM:
OAM datasets do not have an internal record structure; they are accessed as binary data streams.
OAM datasets are not directly cataloged. Rather, they are stored into OAM collections, with only the OAM collection being cataloged. The reason for this is to prevent the catalog from being overloaded with large numbers of (e.g. image) files.
OAM is used in conjunction with DB2. An example use case for OAM would be storing medical images in a DB2 database running under z/OS.