Is there any implementation of AO* algorithm?
I cannot point you towards an existing implementation, but I am pretty confident that it is not too hard to implement yourself. I suggest you to have a look at this paper by Nau, Kumar and Kanal, which has detailed pseudocode for AO* and other related algorithms such as A* and branch and bound. In particular, have a look into Section 7 of that paper and the procedures they call P8 and P9.