diff --git a/nmap.cc b/nmap.cc
index 16daad32b..7103950cc 100644
--- a/nmap.cc
+++ b/nmap.cc
@@ -232,7 +232,9 @@ printf("%s %s ( %s )\n"
        "  --version-all: Try every single probe (intensity 9)\n"
        "  --version-trace: Show detailed version scan activity (for debugging)\n"
        "OS DETECTION:\n"
-       "  -O: Enable OS detection\n"
+       "  -O: Enable OS detection (try 2nd generation, then 1st if that fails)\n"
+       "  -O1: Only use the old (1st generation) OS detection system\n"
+       "  -O2: Only use the new OS detection system (no fallback)\n"
        "  --osscan-limit: Limit OS detection to promising targets\n"
        "  --osscan-guess: Guess OS more aggressively\n"
        "TIMING AND PERFORMANCE:\n"