Because oregano is the closest plant relative to marjoram, it can certainly be substituted for it in certain recipes. Although you will find that these two plants are very closely related, they do have two very distinctive tastes. You will want to keep that in mind when you are trying to decide whether or not to substitute oregano for marjoram.
If you are planning on substituting oregano for marjoram, you should make sure to use only 2/3 the amount that is called for. This way you will be able to get the proportions just right for your recipe so the taste will be nice and even.