While mapping our source conditions to OMOP I have discovered many valid SNOMED codes that are not in OMOP. There are thousands of codes, but I only tested the following:
316931000119104 Pain in right knee (finding)
316791000119102 Pain in left knee (finding)
13211000119109 Lesion of vertebra (finding)
316921000119102 Pain in right hip joint (finding)
451421000124102 Disorder of right lower extremity (disorder)
Note: "Pain in right knee" does match to concept_id = 45596549 (ICD10CM M25.561) in Atlas, but this is a non-standard concept. It maps to the standard concept_id = 4150062 Knee pain. It's a less granular concept.