Guys the whole discussion and ALL your comments are not helpful and absolutely zero constructive! He whants to know how to convert the meshen and not if it legal or if it grey zone. Of cource you may convert everything for your personal use!
According to what Dogsbody wrote above: Making modifications to
any commercial game meshes and using them in Oblivion is, to put it succinctly, against Oblivion's EULA. Moving / exporting meshes between games, no matter if they those games were made by Bethesda or not, is illegal. Period. Even the discussion of how to convert commercial meshes between games is, if not strictly illegal, a bannable offense on these forums, as Dogsbody stated.
Even if people only convert user-made meshes from Morrowind to Oblivion or vice-versa, there is still the issue of the original author's permission. Maybe the original modder doesn't want their meshes imported into Oblivion or doesn't want them to be modified in any way. I know for a fact that many modders get incensed about their mods being put into mod compilations - while the situation may be different, the underlying issues are not: permissions. Unless the modder explicitly gives permission for people to convert / import their meshes / scripts / sounds into Oblivion, then the right thing to do is at least
ask for permission before you do anything regarding
someone else's work. Simply because individual modders don't have a team of lawyers backing them up is beside the point.