Okay, this is a crazy issue, and I'm surprised no one has reported it in so many years! I think this issue has likely existed for 15 years if not more.

The problem is simple:
We pass the file descriptor of the file to which the downloaded document is saved in order to parse it for any links. So, when you combine -p with -O-, what you get is that Wget waits indefinitely on a read() call on stdout. Which will NEVER return anything.

In order to handle -p and -O properly, I think we should download to a temporary file before parsing and then append it to the output file. This is especially important when using -O-. When writing to a real file, we could probably optimize the process by writing directly to the file and then calling fseek() to only process the newly downloaded part of the file.

I would wager this is also an issue with -r -O-.