diff --git a/lib/irc/mode.ml b/lib/irc/mode.ml
index bafb130..c1a4991 100644
--- a/lib/irc/mode.ml
+++ b/lib/irc/mode.ml
@@ -100,13 +100,15 @@ module Set = struct
   }
 
   let pp_change ppf { add; rem } =
-    if rem = 0 then
+    if rem = empty then
       Format.fprintf ppf "+%a" pp add
-    else if add = 0 then
+    else if add = empty then
       Format.fprintf ppf "-%a" pp rem
     else
       Format.fprintf ppf "+%a-%a" pp add pp rem
 
+  let no_change = { add = empty; rem = empty }
+
   let normalize t { add; rem } =
     let add = diff add t in
     let rem = inter rem t in
@@ -120,6 +122,8 @@ module Parse = struct
   let fail fmt =
     Format.kasprintf (fun _ -> raise Error) fmt
 
+  type user_modes = Set.change
+
   let parse_mode_set str ~of_char ~add ~rem ~init =
     let rec loop dir acc i =
       if i >= String.length str then acc
diff --git a/lib/irc/mode.mli b/lib/irc/mode.mli
index f0d8f09..a5ee4c0 100644
--- a/lib/irc/mode.mli
+++ b/lib/irc/mode.mli
@@ -64,19 +64,14 @@ module Set : sig
 
   val pp_change : Format.formatter -> change -> unit
 
+  val no_change : change
   val normalize : t -> change -> t * change
 end
 
 module Parse : sig
   exception Error
 
-  val user_modes : string -> Set.change
+  type user_modes = Set.change
 
-  (* type ('a, 'b) add_rem = Add of 'a | Rem of 'b *)
-  (* type chan_mode_set = *)
-  (*   | A of chan_a * (string, string option) add_rem *)
-  (*   | B of chan_b * (string, string) add_rem *)
-  (*   | C of chan_c * (string, unit) add_rem *)
-  (*   | D of chan_d * (unit, unit) add_rem *)
-  (* val chan : string -> chan_mode_set list *)
+  val user_modes : string -> user_modes
 end
diff --git a/lib/server/connection.ml b/lib/server/connection.ml
index 3a924f7..5e2fd22 100644
--- a/lib/server/connection.ml
+++ b/lib/server/connection.ml
@@ -75,6 +75,8 @@ let notregistered         = "451", ["You have not registered"]
 let needmoreparams cmd    = "461", [cmd; "Not enough parameters"]
 let alreadyregistered     = "462", ["Unauthorized command (already registered)"]
 let modeunknownflag       = "501", ["Unknown MODE flag"]
+let usersdontmatch_set    = "502", ["Can't change mode for other users"]
+let usersdontmatch_get    = "502", ["Can't view mode for other users"]
 
 
 (* user registration *)
@@ -232,21 +234,28 @@ let on_msg_part t name =
 
 (* modes *)
 
-let set_user_mode from user chg =
-  let mode, chg = Irc.Mode.Set.normalize (User.mode user) chg in
-  let modestr = Fmt.str "%a" Irc.Mode.Set.pp_change chg in
-  let msg = Irc.Msg.make "MODE" [User.nick user; modestr] ~always_trailing:true in
-  Router.relay msg ~from [`to_user user; `to_self];
-  User.set_mode user mode
+let set_user_mode user chg =
+  let mode, chg =
+    Irc.Mode.Set.normalize
+      (User.mode user)
+      Irc.Mode.Set.{ chg with add = remove `o chg.add (* can't set +o *) }
+  in
+  if chg <> Irc.Mode.Set.no_change then
+    let modestr = Fmt.str "%a" Irc.Mode.Set.pp_change chg in
+    let msg = Irc.Msg.make "MODE" [User.nick user; modestr] ~always_trailing:true in
+    begin
+      Router.relay msg ~from:user [`to_self];
+      User.set_mode user mode;
+    end
 
-let on_get_user_mode user _me =
-  (* TODO: only +o can get/set modes for users besides themselves *)
+let on_get_user_mode user me =
+  let* () = if user != me then Error usersdontmatch_get else Ok () in
   Ok [
-    "221", [Fmt.str "+%a" Irc.Mode.Set.pp (User.mode user)]
+    "221", [Fmt.str "+%a" Irc.Mode.Set.pp (User.mode me)]
   ]
 
 let on_set_user_mode user me modestr _args =
-  (* TODO: only +o can get/set modes for users besides themselves *)
+  let* () = if user != me then Error usersdontmatch_set else Ok () in
   let* chg = try Ok (Irc.Mode.Parse.user_modes modestr)
     with Irc.Mode.Parse.Error ->
       (* TODO: "If one or more modes sent are not implemented on the server, the server
@@ -254,8 +263,7 @@ let on_set_user_mode user me modestr _args =
          (501) in reply along with the MODE message." *)
       Error modeunknownflag
   in
-  (* TODO: only +o can set +o mode *)
-  set_user_mode me user chg;
+  set_user_mode me chg;
   Ok ()
 
 let on_get_chan_mode chan me =
@@ -319,7 +327,7 @@ let motd t =
 let welcome t me =
   about t me;
   motd t;
-  set_user_mode me me {
+  set_user_mode me {
     add = Irc.Mode.Set.of_string initial_user_modestr;
     rem = Irc.Mode.Set.empty;
   }