Below is the API for the OCaml standard library. It's directly copied over from the OCaml Manual, formatted to the Reason syntax and styled accordingly. The API docs are work-in-progress; we'll be polishing these gradually!
If you're targeting JavaScript, the API docs for BuckleScript includes all of below, plus JS-specific APIs.
sig val mk : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.expression_desc -> Parsetree.expression val attr : Parsetree.expression -> Parsetree.attribute -> Parsetree.expression val ident : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Ast_helper.lid -> Parsetree.expression val constant : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Asttypes.constant -> Parsetree.expression val let_ : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Asttypes.rec_flag -> Parsetree.value_binding list -> Parsetree.expression -> Parsetree.expression val fun_ : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Asttypes.label -> Parsetree.expression option -> Parsetree.pattern -> Parsetree.expression -> Parsetree.expression val function_ : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.case list -> Parsetree.expression val apply : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.expression -> (Asttypes.label * Parsetree.expression) list -> Parsetree.expression val match_ : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.expression -> Parsetree.case list -> Parsetree.expression val try_ : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.expression -> Parsetree.case list -> Parsetree.expression val tuple : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.expression list -> Parsetree.expression val construct : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Ast_helper.lid -> Parsetree.expression option -> Parsetree.expression val variant : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Asttypes.label -> Parsetree.expression option -> Parsetree.expression val record : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> (Ast_helper.lid * Parsetree.expression) list -> Parsetree.expression option -> Parsetree.expression val field : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.expression -> Ast_helper.lid -> Parsetree.expression val setfield : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.expression -> Ast_helper.lid -> Parsetree.expression -> Parsetree.expression val array : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.expression list -> Parsetree.expression val ifthenelse : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.expression -> Parsetree.expression -> Parsetree.expression option -> Parsetree.expression val sequence : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.expression -> Parsetree.expression -> Parsetree.expression val while_ : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.expression -> Parsetree.expression -> Parsetree.expression val for_ : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.pattern -> Parsetree.expression -> Parsetree.expression -> Asttypes.direction_flag -> Parsetree.expression -> Parsetree.expression val coerce : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.expression -> Parsetree.core_type option -> Parsetree.core_type -> Parsetree.expression val constraint_ : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.expression -> Parsetree.core_type -> Parsetree.expression val send : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.expression -> string -> Parsetree.expression val new_ : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Ast_helper.lid -> Parsetree.expression val setinstvar : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Ast_helper.str -> Parsetree.expression -> Parsetree.expression val override : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> (Ast_helper.str * Parsetree.expression) list -> Parsetree.expression val letmodule : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Ast_helper.str -> Parsetree.module_expr -> Parsetree.expression -> Parsetree.expression val assert_ : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.expression -> Parsetree.expression val lazy_ : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.expression -> Parsetree.expression val poly : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.expression -> Parsetree.core_type option -> Parsetree.expression val object_ : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.class_structure -> Parsetree.expression val newtype : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> string -> Parsetree.expression -> Parsetree.expression val pack : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.module_expr -> Parsetree.expression val open_ : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Asttypes.override_flag -> Ast_helper.lid -> Parsetree.expression -> Parsetree.expression val extension : ?loc:Ast_helper.loc -> ?attrs:Ast_helper.attrs -> Parsetree.extension -> Parsetree.expression val case : Parsetree.pattern -> ?guard:Parsetree.expression -> Parsetree.expression -> Parsetree.case end