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.

For convenience, I listed the sequences we've used:

Symmetric Operators

   O11 AATTGTGAGC GCTCACAATT
   O22 AATTGTAAGC GCTTACAATT
   O33 AATTGTAAAC GTTTACAATT
   O44 AATTGTGAAC GTTCACAATT
   O55 AATTTTGAGC GCTCAAAATT
   O66 AATTATGAGC GCTCATAATT
   O77 GACGACTGTA TACAGTCGTC

Asymmetric Operators

"Ox6", "Ox7" and "O6x"

   O16 AATTGTGAGC GCTCATAATT
   O26 AATTGTAAGC GCTCATAATT
   O46 AATTGTGAAC GCTCATAATT
   O61 AATTATGAGC GCTCACAATT
   O17 AATTGTGAGC TACAGTCGTC
   O27 AATTGTAAGC TACAGTCGTC
   O47 AATTGTGAAC TACAGTCGTC

"Oxy"

   O12 AATTGTGAGC GCTTACAATT
   O21 AATTGTAAGC GCTCACAATT
   O24 AATTGTAAGC GTTCACAATT