Have not heard of Osmolax, but we use Lactulose here.

DH had it with his bowel surgery and our GP's have advised it is fine for DD2 to take as she tends to suffer from constipation. It is quite sweet, we just mix it with a bit of water to thin it down. It works on introducing water back to the bowel and just making everything soft. It is not instant, but quite effective if kept up.

Must check out that Osmolax though.