There are tens of thousands of decals available covering a multitude of different models from trains, planes and...
Yes, you can use HO figures on your OO scale layout, there is a slight difference in size, but this is negligible in the grand scheme of things. HO gauge is 1/87th that is 3.5mm to the foot, where as OO scale is 1/76th, that is 4mm to the foot....