diff --git a/kernel/os/include/os/os_callout.h b/kernel/os/include/os/os_callout.h
index 2002de734c..cd65a75ff3 100644
--- a/kernel/os/include/os/os_callout.h
+++ b/kernel/os/include/os/os_callout.h
@@ -128,7 +128,22 @@ os_callout_queued(struct os_callout *c)
  * @cond INTERNAL_HIDDEN
  */
 
+/**
+ * This function is called by the OS in the time tick. It searches the list
+ * of callouts, and sees if any of them are ready to run. If they are ready
+ * to run, it posts an event for each callout that's ready to run,
+ * to the event queue provided to os_callout_init().
+ */
 void os_callout_tick(void);
+
+/**
+ * Returns the number of ticks to the first pending callout. If there are no
+ * pending callouts then return OS_TIMEOUT_NEVER instead.
+ *
+ * @param now               The time now
+ *
+ * @return                  Number of ticks to first pending callout
+ */
 os_time_t os_callout_wakeup_ticks(os_time_t now);
 
 /**
diff --git a/kernel/os/src/os_callout.c b/kernel/os/src/os_callout.c
index 0526a4c91a..18d4db6f74 100644
--- a/kernel/os/src/os_callout.c
+++ b/kernel/os/src/os_callout.c
@@ -114,12 +114,6 @@ os_callout_reset(struct os_callout *c, os_time_t ticks)
 }
 
 
-/**
- * This function is called by the OS in the time tick.  It searches the list
- * of callouts, and sees if any of them are ready to run.  If they are ready
- * to run, it posts an event for each callout that's ready to run,
- * to the event queue provided to os_callout_init().
- */
 void
 os_callout_tick(void)
 {
@@ -158,14 +152,6 @@ os_callout_tick(void)
     os_trace_api_ret(OS_TRACE_ID_CALLOUT_TICK);
 }
 
-/*
- * Returns the number of ticks to the first pending callout. If there are no
- * pending callouts then return OS_TIMEOUT_NEVER instead.
- *
- * @param now The time now
- *
- * @return Number of ticks to first pending callout
- */
 os_time_t
 os_callout_wakeup_ticks(os_time_t now)
 {