OX codes are unique identifiers for geocaches.
All OX codes start with OX and are follow by 1 to 5 letters and numbers. The second part of the code can contain any letters or numbers except letters I, L, O, S, and U. When the first geocache in the OpenCaching.com database was assigned code OXZZZZZ, the next OXZZZZY, counting down to OXZZZZ1, followed OXZZZYZ, OXZZZYY, OXZZZYX and so on.
Some caches that already had ID's in other database before OpenCaching.com launched will be allowed to maintain backwards compatibility by requesting an OX code that closely matches the ID the cache already had elsewhere. For example, if your cache is called XXUI8R in another database, it can be assigned OXUI8R in the OpenCaching.com database.