It would seem that questions about the use of OpenCV should be on topic, because they are usually questions about computer vision algorithms in disguise. There are plenty of such questions on StackOverflow, and I think that they belong here.
I have looked at the last few openCV questions. Most of them seem to be about compiling or use of functions, which would be off-topic for us.
Personally, I feel that that third question should be on-topic for DSP, even if the user needs a solution in a specific language.
Programming wih OpenCV is off-topic. That's purely SO. Questions about how OpenCV algorithms work are on topic. In general, OpenCV questions will be of type Which OpenCV function can I use to achieve effect X?. That's a StackOverflow question. OpenCV is an API, not a particular computer vision algorithm or idea, so most questions will be programming-based.