An osteochondral graft is plug of bone and cartilage that can be press-fitted into a prepared hole to replace a defect in the joint cartilage.

A defect in the cartilage is prepared for grafting by coring out bone and cartilage.

A plug of healthy bone and cartilage is press-fitted into the hole to replace the original tissues.
Osteochondral autograft or allograft?
For smaller defects autograft can be used, where the plug of cartilage and bone is harvested from the patient rather than a donor. Small plugs are harvested along the outer edges of the groove where there is less weight being borne. If the defect is too large for a single plug (OATS) then smaller plugs can be packed like a mosaic into the prepared area (mosaicplasty). For larger defects, cadaver femur can be purchased (pre-sterilised) and the plugs can be taken from that source (allograft).
FORUM DISCUSSIONS
Osteochondral Transplant Allograft Blog.
Possible failed femoral osteochondral allograft - what to do next?
CLINICAL PUBLICATIONS
MR imaging of osteochondral grafts and autologous chondrocyte implantation. Trattnig S, Millington SA, Szomolanyi P and Marlovits S. Eur Radiol. 2007 Jan; 17(1): 103–118.
Knee salvage procedures: The indications, techniques and outcomes of large osteochondral allografts. Chui K, Jeys L and Snow M. World J Orthop. 2015 Apr 18; 6(3): 340–350.