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 any record structure, they are binary streams
OAM datasets are not directly catalogued. Rather, they are stored into OAM collections, with only the OAM collection being catalogued. The reason for this is to prevent the catalogue from being overloaded with large number of e.g. image files.
OAM is meant to be used in conjunction with DB2. An example use case for OAM would be storing medical images in a DB2 database running under z/OS.