There are no Obj.magic function in the OCaml language.
The function is specific to the OCaml compiler implementation, and by using it you are stepping outside of any guarantees provided by the language. In theory, any new versions of the compiler may break any uses of Obj.magic. This does not happen that frequently, but this happens from time to time (the last time this has happened (due to a bug in a corner case of the type system) will be in one month for the 5.3 release).