Are dildos oh Kay to use in the anus?

As long as you are sensible about it. Lubrication is a must. Size does matter, you have to start small and work your way up. Being too aggressive can cause tears and going too deep could become a real health issue. In movies, you can see some who can take 6 inch diameter or an arm almost to the elbow. That took time and they have pretty much ruined themselves from every being normal as far as function goes. Play sensibly and be reasonable and there shouldn't be much of an issue.