diff --git a/oidset.c b/oidset.c
index 9836d427ef..fe4eb921df 100644
--- a/oidset.c
+++ b/oidset.c
@@ -1,6 +1,13 @@
 #include "cache.h"
 #include "oidset.h"
 
+void oidset_init(struct oidset *set, size_t initial_size)
+{
+	memset(&set->set, 0, sizeof(set->set));
+	if (initial_size)
+		kh_resize_oid(&set->set, initial_size);
+}
+
 int oidset_contains(const struct oidset *set, const struct object_id *oid)
 {
 	khiter_t pos = kh_get_oid(&set->set, *oid);
diff --git a/oidset.h b/oidset.h
index 4b90540cd4..c9d0f6d3cc 100644
--- a/oidset.h
+++ b/oidset.h
@@ -38,12 +38,13 @@ struct oidset {
 #define OIDSET_INIT { { 0 } }
 
 
-static inline void oidset_init(struct oidset *set, size_t initial_size)
-{
-	memset(&set->set, 0, sizeof(set->set));
-	if (initial_size)
-		kh_resize_oid(&set->set, initial_size);
-}
+/**
+ * Initialize the oidset structure `set`.
+ *
+ * If `initial_size` is bigger than 0 then preallocate to allow inserting
+ * the specified number of elements without further allocations.
+ */
+void oidset_init(struct oidset *set, size_t initial_size);
 
 /**
  * Returns true iff `set` contains `oid`.