diff --git a/src/lerobot/rl/data_sources/data_mixer.py b/src/lerobot/rl/data_sources/data_mixer.py
index 01c9055be..bdf667c6c 100644
--- a/src/lerobot/rl/data_sources/data_mixer.py
+++ b/src/lerobot/rl/data_sources/data_mixer.py
@@ -89,6 +89,26 @@ class OnlineOfflineMixer(DataMixer):
         async_prefetch: bool = True,
         queue_size: int = 2,
     ):
-        """Yield batches from online/offline mixed sampling."""
+        """Yield batches by composing buffer async iterators."""
+
+        n_online = max(1, int(batch_size * self.online_ratio))
+
+        online_iter = self.online_buffer.get_iterator(
+            batch_size=n_online,
+            async_prefetch=async_prefetch,
+            queue_size=queue_size,
+        )
+
+        if self.offline_buffer is None:
+            yield from online_iter
+            return
+
+        n_offline = batch_size - n_online
+        offline_iter = self.offline_buffer.get_iterator(
+            batch_size=n_offline,
+            async_prefetch=async_prefetch,
+            queue_size=queue_size,
+        )
+
         while True:
-            yield self.sample(batch_size)
+            yield concatenate_batch_transitions(next(online_iter), next(offline_iter))