diff --git a/interop-testing/src/main/java/io/grpc/testing/integration/AbstractInteropTest.java b/interop-testing/src/main/java/io/grpc/testing/integration/AbstractInteropTest.java
index a9c47fbc4d..f69a2a9c64 100644
--- a/interop-testing/src/main/java/io/grpc/testing/integration/AbstractInteropTest.java
+++ b/interop-testing/src/main/java/io/grpc/testing/integration/AbstractInteropTest.java
@@ -117,6 +117,7 @@ import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.ArrayBlockingQueue;
+import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.Executors;
 import java.util.concurrent.LinkedBlockingQueue;
 import java.util.concurrent.ScheduledExecutorService;
@@ -1759,37 +1760,67 @@ public abstract class AbstractInteropTest {
    */
   public void testOrcaOob() throws Exception {
     AtomicReference<TestOrcaReport> reportHolder = new AtomicReference<>();
-    TestOrcaReport answer = TestOrcaReport.newBuilder()
+    final TestOrcaReport answer = TestOrcaReport.newBuilder()
         .setCpuUtilization(0.8210)
         .setMemoryUtilization(0.5847)
         .putUtilization("util", 0.30499)
         .build();
-    blockingStub.unaryCall(SimpleRequest.newBuilder().setOrcaOobReport(answer).build());
-    int i;
-    int retryLimit = 5;
-    for (i = 0; i < retryLimit; i++) {
-      Thread.sleep(1000);
-      blockingStub.withOption(ORCA_OOB_REPORT_KEY, reportHolder).emptyCall(EMPTY);
-      if (reportHolder.get().equals(answer)) {
-        break;
-      }
-    }
-    assertThat(i).isLessThan(retryLimit);
-
-    answer = TestOrcaReport.newBuilder()
+    final TestOrcaReport answer2 = TestOrcaReport.newBuilder()
         .setCpuUtilization(0.29309)
         .setMemoryUtilization(0.2)
         .putUtilization("util", 100.2039)
         .build();
-    blockingStub.unaryCall(SimpleRequest.newBuilder().setOrcaOobReport(answer).build());
-    for (i = 0; i < retryLimit; i++) {
+
+    final int retryLimit = 5;
+    BlockingQueue<Object> queue = new LinkedBlockingQueue<>();
+    final Object lastItem = new Object();
+    StreamObserver<StreamingOutputCallRequest> streamObserver =
+        asyncStub.fullDuplexCall(new StreamObserver<StreamingOutputCallResponse>() {
+
+          @Override
+          public void onNext(StreamingOutputCallResponse value) {
+            queue.add(value);
+          }
+
+          @Override
+          public void onError(Throwable t) {
+            queue.add(t);
+          }
+
+          @Override
+          public void onCompleted() {
+            queue.add(lastItem);
+          }
+        });
+
+    streamObserver.onNext(StreamingOutputCallRequest.newBuilder()
+        .setOrcaOobReport(answer)
+        .addResponseParameters(ResponseParameters.newBuilder().setSize(1).build()).build());
+    assertThat(queue.take()).isInstanceOf(StreamingOutputCallResponse.class);
+    int i = 0;
+    for (; i < retryLimit; i++) {
       Thread.sleep(1000);
       blockingStub.withOption(ORCA_OOB_REPORT_KEY, reportHolder).emptyCall(EMPTY);
-      if (reportHolder.get().equals(answer)) {
+      if (answer.equals(reportHolder.get())) {
         break;
       }
     }
     assertThat(i).isLessThan(retryLimit);
+    streamObserver.onNext(StreamingOutputCallRequest.newBuilder()
+        .setOrcaOobReport(answer2)
+        .addResponseParameters(ResponseParameters.newBuilder().setSize(1).build()).build());
+    assertThat(queue.take()).isInstanceOf(StreamingOutputCallResponse.class);
+
+    for (i = 0; i < retryLimit; i++) {
+      Thread.sleep(1000);
+      blockingStub.withOption(ORCA_OOB_REPORT_KEY, reportHolder).emptyCall(EMPTY);
+      if (reportHolder.get().equals(answer2)) {
+        break;
+      }
+    }
+    assertThat(i).isLessThan(retryLimit);
+    streamObserver.onCompleted();
+    assertThat(queue.take()).isSameInstanceAs(lastItem);
   }
 
   /** Sends a large unary rpc with service account credentials. */
diff --git a/interop-testing/src/main/java/io/grpc/testing/integration/TestServiceImpl.java b/interop-testing/src/main/java/io/grpc/testing/integration/TestServiceImpl.java
index 7d9c4777bf..923aca75f3 100644
--- a/interop-testing/src/main/java/io/grpc/testing/integration/TestServiceImpl.java
+++ b/interop-testing/src/main/java/io/grpc/testing/integration/TestServiceImpl.java
@@ -25,6 +25,7 @@ import io.grpc.ServerCall;
 import io.grpc.ServerCallHandler;
 import io.grpc.ServerInterceptor;
 import io.grpc.Status;
+import io.grpc.StatusRuntimeException;
 import io.grpc.internal.LogExceptionRunnable;
 import io.grpc.services.CallMetricRecorder;
 import io.grpc.services.MetricRecorder;
@@ -50,6 +51,7 @@ import java.util.Random;
 import java.util.Set;
 import java.util.concurrent.Future;
 import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.Semaphore;
 import java.util.concurrent.TimeUnit;
 import javax.annotation.concurrent.GuardedBy;
 
@@ -63,6 +65,7 @@ public class TestServiceImpl extends TestServiceGrpc.TestServiceImplBase {
   private final ScheduledExecutorService executor;
   private final ByteString compressableBuffer;
   private final MetricRecorder metricRecorder;
+  final Semaphore lock = new Semaphore(1);
 
   /**
    * Constructs a controller using the given executor for scheduling response stream chunks.
@@ -126,9 +129,6 @@ public class TestServiceImpl extends TestServiceGrpc.TestServiceImplBase {
     if (req.hasOrcaPerQueryReport()) {
       echoCallMetricsFromPayload(req.getOrcaPerQueryReport());
     }
-    if (req.hasOrcaOobReport()) {
-      echoMetricsFromPayload(req.getOrcaOobReport());
-    }
     responseObserver.onNext(responseBuilder.build());
     responseObserver.onCompleted();
   }
@@ -202,9 +202,29 @@ public class TestServiceImpl extends TestServiceGrpc.TestServiceImplBase {
   public StreamObserver<Messages.StreamingOutputCallRequest> fullDuplexCall(
       final StreamObserver<Messages.StreamingOutputCallResponse> responseObserver) {
     final ResponseDispatcher dispatcher = new ResponseDispatcher(responseObserver);
-    return new StreamObserver<StreamingOutputCallRequest>() {
+    ServerCallStreamObserver<Messages.StreamingOutputCallResponse> autoUnlockResponseObserver =
+        (ServerCallStreamObserver<Messages.StreamingOutputCallResponse>) responseObserver;
+
+    class MayBlockStreamObserver implements StreamObserver<StreamingOutputCallRequest> {
+      boolean oobTestLocked;
+
       @Override
       public void onNext(StreamingOutputCallRequest request) {
+        // to facilitate multiple clients running orca_oob test in parallel, the server allows
+        // only one orca_oob test client to run at a time to avoid conflicts.
+        if (request.hasOrcaOobReport()) {
+          if (!oobTestLocked) {
+            try {
+              lock.acquire();
+            } catch (InterruptedException ex) {
+              autoUnlockResponseObserver.onError(new StatusRuntimeException(
+                  Status.ABORTED.withDescription("server service interrupted").withCause(ex)));
+              return;
+            }
+            oobTestLocked = true;
+          }
+          echoMetricsFromPayload(request.getOrcaOobReport());
+        }
         if (request.hasResponseStatus()) {
           dispatcher.cancel();
           dispatcher.onError(Status.fromCodeValue(request.getResponseStatus().getCode())
@@ -227,7 +247,19 @@ public class TestServiceImpl extends TestServiceGrpc.TestServiceImplBase {
       public void onError(Throwable cause) {
         dispatcher.onError(cause);
       }
-    };
+
+      void cleanup() {
+        if (oobTestLocked) {
+          lock.release();
+          oobTestLocked = false;
+        }
+      }
+    }
+
+    MayBlockStreamObserver mayBlockObserver = new MayBlockStreamObserver();
+    autoUnlockResponseObserver.setOnCancelHandler(mayBlockObserver::cleanup);
+    autoUnlockResponseObserver.setOnCloseHandler(mayBlockObserver::cleanup);
+    return mayBlockObserver;
   }
 
   /**
diff --git a/interop-testing/src/main/proto/grpc/testing/messages.proto b/interop-testing/src/main/proto/grpc/testing/messages.proto
index 2016115462..fbcb6b4ce9 100644
--- a/interop-testing/src/main/proto/grpc/testing/messages.proto
+++ b/interop-testing/src/main/proto/grpc/testing/messages.proto
@@ -103,9 +103,6 @@ message SimpleRequest {
 
   // If set the server should record this metrics report data for the current RPC.
   TestOrcaReport orca_per_query_report = 11;
-
-  // If set the server should update this metrics report data at the OOB server.
-  TestOrcaReport orca_oob_report = 12;
 }
 
 // Unary response, as configured by the request.
@@ -180,6 +177,9 @@ message StreamingOutputCallRequest {
 
   // Whether server should return a given status
   EchoStatus response_status = 7;
+
+  // If set the server should update this metrics report data at the OOB server.
+  TestOrcaReport orca_oob_report = 8;
 }
 
 // Server-streaming response, as configured by the request and parameters.