def formatting_cid_ocn_clusters(cid_ocn_list):
    """Put cid and ocn pairs into clusters by unique cids. 
    Args:
        cid_ocn_list: list of dict with keys of "cid" and "ocn".
        [{"cid": cid1, "ocn": ocn1}, {"cid": cid1, "ocn": ocn2}, {"cid": cid3, "ocn": ocn3}]
    Returns:
        A dict with key=unique cid, value=list of ocns with the same cid.
        {"cid1": [ocn1, ocn2],
         "cid3", [ocn3]}
    """
    # key: cid, value: list of ocns [ocn1, ocn2]
    cid_ocns_dict = {}

    if cid_ocn_list:
        for cid_ocn in cid_ocn_list:
            cid = cid_ocn.get("cid")
            ocn = cid_ocn.get("ocn")
            if cid in cid_ocns_dict:
                cid_ocns_dict[cid].append(ocn)
            else:
                cid_ocns_dict[cid] = [ocn]

    return cid_ocns_dict