def get_onet_occupation(job_posting):
    """Retrieve the occupation from the job posting

    First checks the custom 'onet_soc_code' key,
    then the standard 'occupationalCategory' key,
    and falls back to the unknown occupation
    """
    return job_posting.get('onet_soc_code', job_posting.get('occupationalCategory', '99-9999.00'))