Yes, there are ways to call C from OCaml. The problems are two-fold:
This process is indirect, as it is with any language calling any other language, and requires special care on our part in dealing with matters such as garbage collection (remembering that OCaml is a garbage-collected language, while C is not)
There is no C driver listed on http://voltdb.com/docs/UsingVoltDB/ChapOtherClientAPI.php
There is a C++ driver, so I suppose we could write an OCaml wrapper for C++ code, but as a general rule, it is much more cumbersome to write C++ wrappers than C wrappers.