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I've been trying to bake the floor in solaris urbino 12 model, but the UV mesh seems odd. (See screenshots). When I do smart UV project to get the UV in order to AO bake, the fussboden texture goes very low quality.
Does that mean that I have to UV unwrap the floor manually?
I'm still not really familiar with blender, so I also wanted to know how do I apply the baked AO map to the texture (or, is my node connection already correct?)
in the normal mapping, the floor texture repeats and the UV-Mesh is much bigger than one texture and can also overlap.
This allowes a high quality with less texture space / file size, but no baking
what i would suggest, to enlarge and repeat the floor texture, in order to allow high quality with baking
you have to add the baking result to another layer in your picture programm, set this layer to multiply in order to bring the shadows to the grey/white mottled floor texture
Thanks for the tip. I think it helped. I created a large 22000x22000 image in paint.net and filled it with enlarged fussboden texture, however for some reason blender baked AO map on 1024x1024 image and now I can't really mix the AO map with the texture as they're in completely different resolution.
That's how it looks in blender.
22 x 22 k pixel??? 
I think, this will break OMSI and you'll never will get to see the floor bc it will be always black.
I would never exceed 4096x4096.
Maybe bake again with your final decision of resolution, that the bake output size matches the texture size?
i would first enlarge the image size to 2048
then copy the floor texture from the corner over and over again until the new bigger size is filled, like seen in my picture from yesterday. So it will keep the resolution, but you have more space for UV and baking.
Funfact: You should keep texture sizes by "power of 2". The size can be rectangular or square, but the pixel should always have 2, 8, 16, 32, 64, 128, 256, 1024, 2048, 4096...
2049x1025 will take the texture size in OMSI of 4096x2048 for example. Just because of one f... pixel more
because it shows me some error that no valid objects are selected.
you should click on your floor object in "object mode", then change to "edit mode" and select the faces, then bake, I think. About some things I don't think so much, they are kind of automatic for me.
photoshop or paint.net
you can use, whatever you have installed. Gimp should be also able to do so. And their must be many, many more programs out there.
I started getting familiar with paint.net some years ago and it gives me everything i need, others have bought photoshop and so on.
you have your unbaked floor (symbolised by me with red-black tile pattern) on the bottom layer
on the second layer you have your baking output (symbolised by me with the white-grey ombre), and set it to muliply or overlap / overlay
unified together you have your shadows on the floor texture
for sure, the texture must be in the according bus folder's texture subfolder.
And the o3d of course in model folder and be written in the cfg file