diff --git a/src/structs.h b/src/structs.h
index a320d70..584d472 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -4789,3 +4789,33 @@
 #endif
     int		cts_vcol;	    // virtual column at current position
 } chartabsize_T;
+
+/*
+ * Argument for the callback function (opt_did_set_cb_T) invoked after an
+ * option value is modified.
+ */
+typedef struct
+{
+    int		os_flags;
+    char_u	*os_varp;		// pointer to the option variable
+
+    // old value of the option (can be a string, number or a boolean)
+    union
+    {
+	long	number;
+	int	boolean;
+	char_u	*string;
+    } os_oldval;
+
+    // new value of the option (can be a string, number or a boolean)
+    union
+    {
+	long	number;
+	int	boolean;
+	char_u	*string;
+    } os_newval;
+
+    // When set by the called function: Stop processing the option further.
+    // Currently only used for boolean options.
+    int		os_doskip;
+} optset_T;