Severl utilities for converting content between Morrowind and Oblivion are made available there. TEStroi in particular looks to be very useful for converting Oblivion to the OpenMW engine.
TEStroi is a utility to help you port (convert) the landmass from any worldspace contained in any mod/master created for Oblivion: Elder Scrolls IV (2006) to Morrowind: Elder Scrolls III (2002). It will automatically make an exact replica of the landmass, vertex normals (for shading), vertex colours (for structure layouts and additional shading) and texture the land in the nearest equivalent Morrowind textures, Oblivion originals, or replacement Oblivion textures (such as Qarl's).
It will also convert all the NPCs (name, race, gender, class, vital stats, skills, AI data) and replace the most common non-magical weapons, clothing and armour back to TES3's nearest equivalents. These NPCs are placed back on the landscape in their original locations (including those in child worldspaces). Support is also given to converting Shivering Isles based mods. It can also convert all sounds, lights, activators and statics and place them back on the landscape; you will have to convert TES4 meshes back to TES3 yourself to make usefulness of the placed statics, naturally.
Options exist to move the entire landmass a specified number of TES3 cells in any direction to avoid conflicts with other mods, or to disable the conversion of land, NPCs or books. Additionally, where textures do not exist in the original landmass, TEStroi can apply any Morrowind/Bloodmoon or custom texture of your choice to the entire landmass, such as snow, grass, rock, lava etc. helping to reduce the effort required to repaste the entire region.
TEStroi will produce a masterless Morrowind compatible plugin called troi_out.esp which may be loaded in to either the Morrowind TES Construction Set (TESCS) or in to the game itself and visited.
Please note this is presently a Console application only, though it is simple enough to use.
You can see some Screenshots of how Oblivion's Tamriel looks when converted with TEStroi by clicking on this link.