In-situ OAM (sometimes also referred to as inband OAM, which was the term originally used before IETF chose "in-situ OAM" for the technology) is a technology to record operational information in the packet while the packet traverses a path between two points in the network. In-band OAM is to complement current out-of-band OAM (sometimes also called "active" OAM) mechanisms based on ICMP or other types of probe packets.
For more information on in-band OAM visit: https://github.com/CiscoDevNet/iOAM