Complex sentence: In the USA most onshore (land) oil and gas rights (OGM) are owned by private individuals, in which case oil companies must negotiate terms for a lease of these rights with the individual who owns the OGM.
Split into simpler sentences: In the USA most onshore (land) oil and gas rights (OGM) are owned by private individuals. In this case oil companies must negotiate terms for a lease of these rights with the individual who owns the OGM.