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 type space_formatter = (unit, Format.formatter, unit) Pervasives.format class printer : unit -> object ('b) val pipe : bool val semi : bool method attribute : Format.formatter -> Parsetree.attribute -> unit method attributes : Format.formatter -> Parsetree.attributes -> unit method binding : Format.formatter -> Parsetree.value_binding -> unit method bindings : Format.formatter -> Asttypes.rec_flag * Parsetree.value_binding list -> unit method case_list : Format.formatter -> Parsetree.case list -> unit method class_expr : Format.formatter -> Parsetree.class_expr -> unit method class_field : Format.formatter -> Parsetree.class_field -> unit method class_params_def : Format.formatter -> (Parsetree.core_type * Asttypes.variance) list -> unit method class_signature : Format.formatter -> Parsetree.class_signature -> unit method class_structure : Format.formatter -> Parsetree.class_structure -> unit method class_type : Format.formatter -> Parsetree.class_type -> unit method class_type_declaration_list : Format.formatter -> Parsetree.class_type_declaration list -> unit method constant : Format.formatter -> Asttypes.constant -> unit method constant_string : Format.formatter -> string -> unit method core_type : Format.formatter -> Parsetree.core_type -> unit method core_type1 : Format.formatter -> Parsetree.core_type -> unit method direction_flag : Format.formatter -> Asttypes.direction_flag -> unit method directive_argument : Format.formatter -> Parsetree.directive_argument -> unit method exception_declaration : Format.formatter -> Parsetree.extension_constructor -> unit method expression : Format.formatter -> Parsetree.expression -> unit method expression1 : Format.formatter -> Parsetree.expression -> unit method expression2 : Format.formatter -> Parsetree.expression -> unit method extension : Format.formatter -> Parsetree.extension -> unit method extension_constructor : Format.formatter -> Parsetree.extension_constructor -> unit method floating_attribute : Format.formatter -> Parsetree.attribute -> unit method item_attribute : Format.formatter -> Parsetree.attribute -> unit method item_attributes : Format.formatter -> Parsetree.attributes -> unit method item_extension : Format.formatter -> Parsetree.extension -> unit method label_exp : Format.formatter -> Asttypes.label * Parsetree.expression option * Parsetree.pattern -> unit method label_x_expression_param : Format.formatter -> Asttypes.label * Parsetree.expression -> unit method list : ?sep:Pprintast.space_formatter -> ?first:Pprintast.space_formatter -> ?last:Pprintast.space_formatter -> (Format.formatter -> 'a -> unit) -> Format.formatter -> 'a list -> unit method longident : Format.formatter -> Longident.t -> unit method longident_loc : Format.formatter -> Longident.t Asttypes.loc -> unit method module_expr : Format.formatter -> Parsetree.module_expr -> unit method module_type : Format.formatter -> Parsetree.module_type -> unit method mutable_flag : Format.formatter -> Asttypes.mutable_flag -> unit method nonrec_flag : Format.formatter -> Asttypes.rec_flag -> unit method option : ?first:Pprintast.space_formatter -> ?last:Pprintast.space_formatter -> (Format.formatter -> 'a -> unit) -> Format.formatter -> 'a option -> unit method paren : ?first:Pprintast.space_formatter -> ?last:Pprintast.space_formatter -> bool -> (Format.formatter -> 'a -> unit) -> Format.formatter -> 'a -> unit method pattern : Format.formatter -> Parsetree.pattern -> unit method pattern1 : Format.formatter -> Parsetree.pattern -> unit method payload : Format.formatter -> Parsetree.payload -> unit method private_flag : Format.formatter -> Asttypes.private_flag -> unit method rec_flag : Format.formatter -> Asttypes.rec_flag -> unit method reset : 'b method reset_ifthenelse : 'b method reset_pipe : 'b method reset_semi : 'b method signature : Format.formatter -> Parsetree.signature_item list -> unit method signature_item : Format.formatter -> Parsetree.signature_item -> unit method simple_expr : Format.formatter -> Parsetree.expression -> unit method simple_pattern : Format.formatter -> Parsetree.pattern -> unit method string_quot : Format.formatter -> Asttypes.label -> unit method structure : Format.formatter -> Parsetree.structure_item list -> unit method structure_item : Format.formatter -> Parsetree.structure_item -> unit method sugar_expr : Format.formatter -> Parsetree.expression -> bool method toplevel_phrase : Format.formatter -> Parsetree.toplevel_phrase -> unit method type_declaration : Format.formatter -> Parsetree.type_declaration -> unit method type_def_list : Format.formatter -> Parsetree.type_declaration list -> unit method type_extension : Format.formatter -> Parsetree.type_extension -> unit method type_param : Format.formatter -> Parsetree.core_type * Asttypes.variance -> unit method type_params : Format.formatter -> (Parsetree.core_type * Asttypes.variance) list -> unit method type_with_label : Format.formatter -> Asttypes.label * Parsetree.core_type -> unit method tyvar : Format.formatter -> string -> unit method under_ifthenelse : 'b method under_pipe : 'b method under_semi : 'b method value_description : Format.formatter -> Parsetree.value_description -> unit method virtual_flag : Format.formatter -> Asttypes.virtual_flag -> unit end val default : Pprintast.printer val toplevel_phrase : Format.formatter -> Parsetree.toplevel_phrase -> unit val expression : Format.formatter -> Parsetree.expression -> unit val string_of_expression : Parsetree.expression -> string val top_phrase : Format.formatter -> Parsetree.toplevel_phrase -> unit val core_type : Format.formatter -> Parsetree.core_type -> unit val pattern : Format.formatter -> Parsetree.pattern -> unit val signature : Format.formatter -> Parsetree.signature -> unit val structure : Format.formatter -> Parsetree.structure -> unit val string_of_structure : Parsetree.structure -> string end