A great way to get free support is by using the active mailing lists. When you need to go beyond this and get professional support, you have the following options:
The OCamlPro Company
OCamlPro is the creator of many open-source tools widely used throughout the community, such as Try OCaml, the OPAM package manager and ocp-indent, as well as a large contributor to OCaml itself. Besides commercially supporting their tools, they offer to share their expertise through full OCaml support packages. They also provide trainings and specialized software developments.
OCamlPro is an INRIA spin-off with a team of highly skilled experienced OCaml programmers, including members of the OCaml core development team, and they have expertise to help debug and optimize OCaml projects as well as improve specific work environments. See details here
Gerd Stolpmann has been helping companies master OCaml since 2005. He is an expert of the ecosystem surrounding OCaml and developed the GODI platform. Stolpmann is a computer scientist who has been a contractor for several long-running OCaml projects. He has a focus on big data (including data preparation, search/query engines, map/reduce), but his skills also cover Unix system programming, SQL databases, client/server, compiler development (e.g. for domain-specific languages), and much more. Also visit his website on OCaml.
You can join the Caml Consortium to support development of the OCaml compiler. See details here.
You can support OCaml Labs, which runs a variety of open source projects to support the OCaml community. See details here.