diff --git a/src/core/pbuf.c b/src/core/pbuf.c
index 4649fb87..285b1508 100644
--- a/src/core/pbuf.c
+++ b/src/core/pbuf.c
@@ -84,18 +84,25 @@
    aligned there. Therefore, PBUF_POOL_BUFSIZE_ALIGNED can be used here. */
 #define PBUF_POOL_BUFSIZE_ALIGNED LWIP_MEM_ALIGN_SIZE(PBUF_POOL_BUFSIZE)
 
-#if !LWIP_TCP || !TCP_QUEUE_OOSEQ || NO_SYS
+#if !LWIP_TCP || !TCP_QUEUE_OOSEQ || !PBUF_POOL_FREE_OOSEQ
 #define PBUF_POOL_IS_EMPTY()
-#else /* !LWIP_TCP || !TCP_QUEUE_OOSEQ || NO_SYS */
-/** Define this to 0 to prevent freeing ooseq pbufs when the PBUF_POOL is empty */
-#ifndef PBUF_POOL_FREE_OOSEQ
-#define PBUF_POOL_FREE_OOSEQ 1
-#endif /* PBUF_POOL_FREE_OOSEQ */
+#else /* !LWIP_TCP || !TCP_QUEUE_OOSEQ || !PBUF_POOL_FREE_OOSEQ */
 
-#if PBUF_POOL_FREE_OOSEQ
+#if !NO_SYS
+#ifndef PBUF_POOL_FREE_OOSEQ_QUEUE_CALL
 #include "lwip/tcpip.h"
+#define PBUF_POOL_FREE_OOSEQ_QUEUE_CALL()  do { \
+  if(tcpip_callback_with_block(pbuf_free_ooseq_callback, NULL, 0) != ERR_OK) { \
+      SYS_ARCH_PROTECT(old_level); \
+      pbuf_free_ooseq_pending = 0; \
+      SYS_ARCH_UNPROTECT(old_level); \
+  } } while(0)
+#endif /* PBUF_POOL_FREE_OOSEQ_QUEUE_CALL */
+#endif /* !NO_SYS */
+
+volatile u8_t pbuf_free_ooseq_pending;
 #define PBUF_POOL_IS_EMPTY() pbuf_pool_is_empty()
-static u8_t pbuf_free_ooseq_queued;
+
 /**
  * Attempt to reclaim some memory from queued out-of-sequence TCP segments
  * if we run out of pool pbufs. It's better to give priority to new packets
@@ -104,15 +111,14 @@ static u8_t pbuf_free_ooseq_queued;
  * This must be done in the correct thread context therefore this function
  * can only be used with NO_SYS=0 and through tcpip_callback.
  */
-static void
-pbuf_free_ooseq(void* arg)
+void
+pbuf_free_ooseq()
 {
   struct tcp_pcb* pcb;
   SYS_ARCH_DECL_PROTECT(old_level);
-  LWIP_UNUSED_ARG(arg);
 
   SYS_ARCH_PROTECT(old_level);
-  pbuf_free_ooseq_queued = 0;
+  pbuf_free_ooseq_pending = 0;
   SYS_ARCH_UNPROTECT(old_level);
 
   for (pcb = tcp_active_pcbs; NULL != pcb; pcb = pcb->next) {
@@ -126,29 +132,39 @@ pbuf_free_ooseq(void* arg)
   }
 }
 
+/**
+ * Just a callback function for tcpip_timeout() that calls pbuf_free_ooseq().
+ */
+static void
+pbuf_free_ooseq_callback(void *arg)
+{
+  LWIP_UNUSED_ARG(arg);
+  pbuf_free_ooseq();
+}
+
 /** Queue a call to pbuf_free_ooseq if not already queued. */
 static void
 pbuf_pool_is_empty(void)
 {
-  u8_t queued;
   SYS_ARCH_DECL_PROTECT(old_level);
-
+#ifndef PBUF_POOL_FREE_OOSEQ_QUEUE_CALL
   SYS_ARCH_PROTECT(old_level);
-  queued = pbuf_free_ooseq_queued;
-  pbuf_free_ooseq_queued = 1;
+  pbuf_free_ooseq_pending = 1;
+  SYS_ARCH_UNPROTECT(old_level);
+#else /* PBUF_POOL_FREE_OOSEQ_QUEUE_CALL */
+  u8_t queued;
+  SYS_ARCH_PROTECT(old_level);
+  queued = pbuf_free_ooseq_pending;
+  pbuf_free_ooseq_pending = 1;
   SYS_ARCH_UNPROTECT(old_level);
 
   if(!queued) {
     /* queue a call to pbuf_free_ooseq if not already queued */
-    if(tcpip_callback_with_block(pbuf_free_ooseq, NULL, 0) != ERR_OK) {
-      SYS_ARCH_PROTECT(old_level);
-      pbuf_free_ooseq_queued = 0;
-      SYS_ARCH_UNPROTECT(old_level);
-    }
+    PBUF_POOL_FREE_OOSEQ_QUEUE_CALL();
   }
+#endif /* PBUF_POOL_FREE_OOSEQ_QUEUE_CALL */
 }
-#endif /* PBUF_POOL_FREE_OOSEQ */
-#endif /* !LWIP_TCP || !TCP_QUEUE_OOSEQ || NO_SYS */
+#endif /* !LWIP_TCP || !TCP_QUEUE_OOSEQ || !PBUF_POOL_FREE_OOSEQ */
 
 /**
  * Allocates a pbuf of the given type (possibly a chain for PBUF_POOL type).
diff --git a/src/core/timers.c b/src/core/timers.c
index c804c3d1..3c4f8d1d 100644
--- a/src/core/timers.c
+++ b/src/core/timers.c
@@ -60,6 +60,7 @@
 #include "lwip/ip6_frag.h"
 #include "lwip/mld6.h"
 #include "lwip/sys.h"
+#include "lwip/pbuf.h"
 
 /** The one and only timeout list */
 static struct sys_timeo *next_timeout;
@@ -428,6 +429,9 @@ sys_check_timeouts(void)
     diff = LWIP_U32_DIFF(now, timeouts_last_time);
     do
     {
+#if PBUF_POOL_FREE_OOSEQ
+      PBUF_CHECK_FREE_OOSEQ();
+#endif /* PBUF_POOL_FREE_OOSEQ */
       had_one = 0;
       tmptimeout = next_timeout;
       if (tmptimeout->time <= diff) {
diff --git a/src/include/lwip/pbuf.h b/src/include/lwip/pbuf.h
index 50407e90..41c49099 100644
--- a/src/include/lwip/pbuf.h
+++ b/src/include/lwip/pbuf.h
@@ -127,6 +127,24 @@ struct pbuf_custom {
 };
 #endif /* LWIP_SUPPORT_CUSTOM_PBUF */
 
+#if LWIP_TCP && TCP_QUEUE_OOSEQ
+/** Define this to 0 to prevent freeing ooseq pbufs when the PBUF_POOL is empty */
+#ifndef PBUF_POOL_FREE_OOSEQ
+#define PBUF_POOL_FREE_OOSEQ 1
+#endif /* PBUF_POOL_FREE_OOSEQ */
+#if NO_SYS && PBUF_POOL_FREE_OOSEQ
+extern volatile u8_t pbuf_free_ooseq_pending;
+void pbuf_free_ooseq();
+/** When not using sys_check_timeouts(), call PBUF_CHECK_FREE_OOSEQ()
+    at regular intervals from main level to check if ooseq pbufs need to be
+    freed! */
+#define PBUF_CHECK_FREE_OOSEQ() do { if(pbuf_free_ooseq_pending) { \
+  /* pbuf_alloc() reported PBUF_POOL to be empty -> try to free some \
+     ooseq queued pbufs now */ \
+  pbuf_free_ooseq(); }}while(0)
+#endif /* NO_SYS && PBUF_POOL_FREE_OOSEQ*/
+#endif /* LWIP_TCP && TCP_QUEUE_OOSEQ */
+
 /* Initializes the pbuf module. This call is empty for now, but may not be in future. */
 #define pbuf_init()