It should work in both the Orthographic and Perspective cameras if you change the vertex program to something like:

v2f vert(appdata v) {
    v2f o;
    o.pos = v.vertex;
    o.pos.xyz += v.normal.xyz *_Outline;
    o.pos = mul(UNITY_MATRIX_MVP, o.pos);
    o.color = _OutlineColor;
    return o;
}

The problem with the old version is that the position was being adjusted in camera-space and The projected normals don't behave the same in orthographic projection as they do in perspective projection. The simple workaround used here is to add the outline in world-space and then project to camera space.