The reason the Oscar quit hiding when the turtle was introduced is the schooling instinct. When Oscars are that small they will generally be in a school. When he got seperated and put alone, he was scared and it was not natural to him, so he would hide. The turtle is the closest thing to a school hes gonna get so he did what came natural. Cichla also tend to be this way when they are young. The Oscar would have also started coming out more if you would have added more fish. Anytime you have a fish that is very shy, just add a couple active fish to the tank, it will give them the confidence they need to get out and move around. Granted those active fish arent small enough to be eaten or too aggressive.