diff --git a/src/derp/bot.py b/src/derp/bot.py
index 40ad43a..6483819 100644
--- a/src/derp/bot.py
+++ b/src/derp/bot.py
@@ -265,6 +265,12 @@ class Bot:
             if "*" in flags:
                 self._opers.add(f"{nick}!{user}@{host}")
 
+        # JOIN — WHO the joining user to detect oper status
+        if msg.command == "JOIN" and msg.nick and msg.nick != self.nick:
+            channel = msg.params[0] if msg.params else ""
+            if channel:
+                await self.conn.send(format_msg("WHO", msg.nick))
+
         # QUIT — remove departed nicks from oper set
         if msg.command == "QUIT" and msg.prefix:
             self._opers.discard(msg.prefix)
diff --git a/tests/test_integration.py b/tests/test_integration.py
index 2bd3897..940e464 100644
--- a/tests/test_integration.py
+++ b/tests/test_integration.py
@@ -292,6 +292,27 @@ class TestAdmin:
         assert not any("Permission denied" in r for r in replies)
         assert any("Opers:" in r for r in replies)
 
+    def test_oper_detection_on_join(self):
+        """User joining a channel triggers WHO; oper detected mid-session."""
+        h = _Harness(channels=["#test"])
+        h.inject_registration()
+        # User joins the channel after the bot is already connected
+        h.conn.inject(":oper!oper@operhost JOIN #test")
+        # Server responds to the WHO triggered by the JOIN
+        h.conn.inject(
+            ":server 352 test #test oper operhost irc.server oper H* "
+            ":0 Oper Name"
+        )
+        h.privmsg("oper", "#test", "!admins", user="oper", host="operhost")
+        asyncio.run(h.run())
+
+        # Verify WHO was sent for the joining nick
+        who_sent = [s for s in h.conn.sent if "WHO" in s and "oper" in s]
+        assert len(who_sent) >= 1
+        replies = h.sent_privmsgs("#test")
+        assert not any("Permission denied" in r for r in replies)
+        assert any("oper!oper@operhost" in r for r in replies)
+
 
 # -- Channel filter ---------------------------------------------------------