Unity - Scripting API: Mesh.normals the document do not say that
That question doesn’t really make sense. Of course the mesh’s vertex normals are in object space. The normals (together with the tangents) form the tangent space. So you can’t define vectors in a space they actually define. The tangent space is only relevant when you do normal mapping. i.e. you apply a normal map texture to a surface. The normals in a normal map are usually defined in tangent space. The vertex normals actually define the tangentspace.