The argument from illusion infers from the fact that we can sometimes be deceived by our perceptions (i.e. suffer from misapprehensions or illusions) that we do not directly perceive the external world.
The argument necessitates that our sense perceptions are "processed" by our minds and our cognitive faculties (in terms of information processing) have a significant role to play in our belief in and our awareness of the "external world".
My position is that the argument from illusion is flawed. It does not necessarily follow from the fact that we are sometimes deceived by our senses that we are always deceived by our senses. Even if it were the case that we cannot tell when we are being deceived (which I think is dubious) this would not mean that we cannot, sometimes, directly perceive the external world.
I would give the example of a camera with a random electronic fault. Sometimes the camera correctly captures an image, at other times it distorts the image so that all the colours are reversed. I do not believe that we would we conclude from the presence of the fault that the camera can never capture a true image, all we could conclude is that we would need to be careful about the images from that camera.
The example of a stick appearing bent under water is also not a good argument for lack of direct perception of the external world. Since we understand why the stick appears bent, I would have thought that the stick appearing straight under water would be a better argument for illusion. How should the stick appear? It should appear bent if we directly perceive the external world.
Similarly with the example of the oasis in the desert. Under the correct atmospheric and psychological conditions we would expect to see an oasis where there was not in fact one to see if we did in fact directly perceive the world.
I think a much stronger argument for illusion would be where someone does in fact see a "miracle", i.e. something which is known to be not of the real world. I am not aware of any such cases so the argument, in my view, fails.