/usr/include/gazebo-9/gazebo/opcode/ is in libgazebo9-dev 9.0.0+dfsg5-3ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/gazebo-9/gazebo/opcode/OPC_AABBCollider.h
/usr/include/gazebo-9/gazebo/opcode/OPC_AABBTree.h
/usr/include/gazebo-9/gazebo/opcode/OPC_BaseModel.h
/usr/include/gazebo-9/gazebo/opcode/OPC_BoxBoxOverlap.h
/usr/include/gazebo-9/gazebo/opcode/OPC_Collider.h
/usr/include/gazebo-9/gazebo/opcode/OPC_Common.h
/usr/include/gazebo-9/gazebo/opcode/OPC_HybridModel.h
/usr/include/gazebo-9/gazebo/opcode/OPC_IceHook.h
/usr/include/gazebo-9/gazebo/opcode/OPC_LSSAABBOverlap.h
/usr/include/gazebo-9/gazebo/opcode/OPC_LSSCollider.h
/usr/include/gazebo-9/gazebo/opcode/OPC_LSSTriOverlap.h
/usr/include/gazebo-9/gazebo/opcode/OPC_MeshInterface.h
/usr/include/gazebo-9/gazebo/opcode/OPC_Model.h
/usr/include/gazebo-9/gazebo/opcode/OPC_OBBCollider.h
/usr/include/gazebo-9/gazebo/opcode/OPC_OptimizedTree.h
/usr/include/gazebo-9/gazebo/opcode/OPC_Picking.h
/usr/include/gazebo-9/gazebo/opcode/OPC_PlanesAABBOverlap.h
/usr/include/gazebo-9/gazebo/opcode/OPC_PlanesCollider.h
/usr/include/gazebo-9/gazebo/opcode/OPC_PlanesTriOverlap.h
/usr/include/gazebo-9/gazebo/opcode/OPC_RayAABBOverlap.h
/usr/include/gazebo-9/gazebo/opcode/OPC_RayCollider.h
/usr/include/gazebo-9/gazebo/opcode/OPC_RayTriOverlap.h
/usr/include/gazebo-9/gazebo/opcode/OPC_Settings.h
/usr/include/gazebo-9/gazebo/opcode/OPC_SphereAABBOverlap.h
/usr/include/gazebo-9/gazebo/opcode/OPC_SphereCollider.h
/usr/include/gazebo-9/gazebo/opcode/OPC_SphereTriOverlap.h
/usr/include/gazebo-9/gazebo/opcode/OPC_TreeBuilders.h
/usr/include/gazebo-9/gazebo/opcode/OPC_TreeCollider.h
/usr/include/gazebo-9/gazebo/opcode/OPC_TriBoxOverlap.h
/usr/include/gazebo-9/gazebo/opcode/OPC_TriTriOverlap.h
/usr/include/gazebo-9/gazebo/opcode/OPC_VolumeCollider.h
/usr/include/gazebo-9/gazebo/opcode/Opcode.h
/usr/include/gazebo-9/gazebo/opcode/Stdafx.h