This works fine for me using more current IM 6 syntax. Note your images were converted from png to jpg upon uploading. Thus any alpha channel in either would have been lost. So if you are using PNG or gif or any image format that supports transparency for either of your image, you need to disable the alpha channel by using -alpha off or the older form +matte. That could be your problem apart from the wrong image order.
convert oql5nnojqmlyivxglah7_thumb.png.jpeg 87es0okco191h7vozvgn_thumb.png.jpeg -alpha off -compose copy_opacity -composite 1tmp1.png
so does this
composite -compose copy_opacity +matte 87es0okco191h7vozvgn_thumb.png.jpeg oql5nnojqmlyivxglah7_thumb.png.jpeg 1tmp2.png
I am using IM 22.214.171.124 Q16 Mac OSX Snow Leopard. You never said what version of IM you are using and what platform. Perhaps you need an upgrade
Look carefully at the syntax at http://www.imagemagick.org/Usage/compose/#compose
The image order needs to be swapped between the convert syntax and the composite syntax. Your mask image is not the same as the "mask" image in the documentation. It is just the overlay image. You only the third image is really the "mask" image