diff --git a/examples/client.c b/examples/client.c
index ca16a584..edfe7982 100644
--- a/examples/client.c
+++ b/examples/client.c
@@ -648,7 +648,9 @@ usage( const char *program, const char *version) {
      "\t       \t\t(default is %d)\n"
      "\t-K interval\tsend a ping after interval seconds of inactivity\n"
      "\t-N     \t\tSend NON-confirmable message\n"
-     "\t-O num,text\tAdd option num with contents text to request\n"
+     "\t-O num,text\tAdd option num with contents text to request. If the\n"
+     "\t       \t\ttext begins with 0x, then the hex text is converted to\n"
+     "\t       \t\tbinary data\n"
      "\t-P addr[:port]\tUse proxy (automatically adds Proxy-Uri option to\n"
      "\t       \t\trequest)\n"
      "\t-T token\tInclude specified token\n"
diff --git a/man/coap-client.txt.in b/man/coap-client.txt.in
index 8f93de6a..f7dcd54c 100644
--- a/man/coap-client.txt.in
+++ b/man/coap-client.txt.in
@@ -117,7 +117,8 @@ OPTIONS - General
    confirmable message will be sent.
 
 *-O* num,text::
-   Add option 'num' with contents of 'text' to the request.
+   Add option 'num' with contents of 'text' to the request. If the text begins
+   with 0x, then the hex text is converted to binary data.
 
 *-P* addr[:port]::
    Address (and port) for proxy to use (automatically adds Proxy-Uri option