From 2007.igem.org

Biologists always use symmetric LacI-family operators, e.g. AATTGTGAAC GTTCACAATT (O44) and AATTGTAAGC GCTTACAATT (O22). Please notice that they are all 20bp long and each right 10bp-long strand’s complementary strand is reversed symmetric with its left 10bp-long strand. For short we call this kinds of symmetrical operators as Oxx, for example, O44, O22, etc. Then you must have already understood what O24 means-- Yes, it means the asymmetric operator AATTGTAAGC GTTCACAATT (O24), which has the left 10bp-long strand of O22 and the right 10bp-long one of O44. Again for example, O42 means AATTGTGAAC GCTTACAATT (O42), and so on. We also give these asymmetric operators another name: Hybrid Operators.