Use PPX Deriving JSON. PPX is a syntax extension and PPX Deriving JSON derives functions when annotating a type to convert from and to JSON.

type person = {
  name: string;
  age: int;
} [@@deriving yojson]

will generate the functions

type person = { name : string; age : int; }
val person_to_yojson : person -> Yojson.Safe.json = <fun>
val person_of_yojson : Yojson.Safe.json -> person Ppx_deriving_yojson_runtime.error_or = <fun>

You can then play with the converters like this:

let x : person = {name = "hello"; age = 15};;
(* val x : person = {name = "hello"; age = 15} *)
person_to_yojson x;;
(* - : Yojson.Safe.json = `Assoc [("name", `String "hello"); ("age", `Int 15)] *)

If you use dune, you’ve got to use this

(library
 ((name test)
  (public_name test)
  (modules (test))
  (libraries (yojson ppx_deriving ppx_deriving.runtime
              ppx_deriving_yojson.runtime))
  (preprocess (pps (ppx_deriving ppx_deriving_yojson)))
))

If the type is called t, the functions are to_json and of_json