Wat Bo, the oldest and largest temple in Provincial town of Siem Reap, is also well known for its old paintings on its Vihara’s wall, depicting the epic of Ramayana (known as the Reamker in Khmer). The murals in the Vihara of Wat Bo and inside Royal Palace in Phnom Penh are said to be the oldest ones in Cambodia, though the latter one is older.
If you visit the temple and find the vihara closed, do not disappointed and find your way back. Try to find the old man with the key. He is kind enough to open the doors and windows for any person wishing to see the paintings. It was a bit dark when I was there, but luckily I brought along my tripod, without which it would be hard to get a decent exposure.
Read more about Wat Bo at expat-advisory