First, make sure your nif exported with a "NiTexturingProperty", with a "NiSourceTexture" under that.
So open up your nif in Nifskope, expand the NiTriStrips or NiTriShape (click on the arrow next to that name). You should then see stuff like NiBinaryExtraData, NiTriStripsData, & NiMaterialProperty. But if there is no NiTexturingProperty, with no NiSourceTexture under it (when you "expand" NiTexturingProperty), then this is the cause of your problem (or one of the causes).
This means you didn't assign a texture and it's texture folder path to the nif, in Blender, so be sure you read this tutorial and see if it helps you:
Blender UVMapping Tutorial, by Koniption
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Secondly, the "NiSourceTexture" should point to the folder path of the dds texture, as it exists in the Oblivion Data folder. So put your color texture (i.e. colortexture.dds) and the associated normal map (i.e. colortexture_n.dds; must be same name as the colored textured, with an "_n.nif" on the end), in the "Data\textures\CustomSubfolder\CustomSubfolder\etc...". "CustomSubfolder" denotes a custom named folder of your choice (you can have one or more of these), but you absolutely must be in the "textures" section of the Data folder.
So open up the nif in Nifskope, go to the NiSourceTexture, click on the "purple flower" icon to the right, and browse to where your color texture is in the "textures" folder of Oblivion, then click on it, hit "Ok". Do not select the normal map which is a blue-hued texture ("_n.nif"), just select the regular colored texture. Be sure to save your nif, after this.
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Thirdly, just about every texture in Oblivion needs a normal map. Make sure you make one. GIMP program has a dds plugin & a normal map plugin, that you can download separately, to make the color textures & normal maps that you need. It's free program. Normal map needs to go into the same folder as the regular color texture, btw, in the "textures" section of Oblivion files.
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Fourthly, the dds textures that Oblivion uses, whether color texture or normal map, needs to be a "power of 2" dimension:
8 x 8
16 x 16
32 x 32
64 x 64
..........etc (Notice how each next level doubles in the number value for length and width?)
Go no bigger than 1024x1024, optimally (for larger or more detailed meshes), but I've heard up to 2048x2048 can also be used, though it doesn't make much of a visible detectable difference (unless you are texturing a mesh that will be very big in the game). Usually, 512 x 512, is suitable for most tasks.
But your dds dimensions don't have to be perfectly square - they can be rectangular in any combination of length and width, too, as long as the "power of 2" dimension rule is still followed:
32 x 64
128 x 512
64 x 1024
..............etc
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If you are still having trouble after following the advice above, please PM me the files you made/exported: the nif, color texture, normal map texture, and Blender scene. You can use Filefront website to upload them, or some other one.
Koniption