A few words of warning about Pnomh Tamao:
It's a fair trek out of town, and the last bit is a long dirt track road full of beggars, so take a taxi not a tuk tuk.
The park itself is pretty big and not particularly well laid out. It's actually a rescue centre rather than a zoo, but they do encourage their own organised tours which are probably better to be honest, although pricey (but for a good cause). On their tours they will come and collect you.
Wildlife Tours ? Wildlife Alliance
I enjoyed it.