diff --git a/protocol/app/flags/flags.go b/protocol/app/flags/flags.go
index b166f88352..1a0ad084e1 100644
--- a/protocol/app/flags/flags.go
+++ b/protocol/app/flags/flags.go
@@ -80,7 +80,7 @@ const (
 
 	DefaultVEOracleEnabled = true
 
-	DefaultOptimisticExecutionEnabled       = false
+	DefaultOptimisticExecutionEnabled       = true
 	DefaultOptimisticExecutionTestAbortRate = 0
 )
 
diff --git a/protocol/testutil/app/app.go b/protocol/testutil/app/app.go
index a4dbac0bc2..69d76021f3 100644
--- a/protocol/testutil/app/app.go
+++ b/protocol/testutil/app/app.go
@@ -1308,6 +1308,9 @@ func launchValidatorInDir(
 		"--oracle.enabled=false",
 		"--oracle.metrics_enabled=false",
 		"--log_level=error",
+		// TODO(CT-1329): Currently, the TestApp framework does not work well with OE,
+		// since the non-deterministic test app instances does not handle go routines.
+		"--optimistic-execution-enabled=false",
 	})
 
 	ctx := svrcmd.CreateExecuteContext(parentCtx)