Most major subscription services allow you to download movies to your device for offline viewing. Here is where Ong Bak is currently available (check your region):
Pro Tip: If you search the phrase but want to stay legal, add “legal stream” or “buy” to your query. For example: “Buy Ong-Bak 2003 720p download”
Let’s address the elephant in the ring. The keyword includes “Download”, which often implies piracy. However, as a responsible content creator, I must distinguish between safe/legal and risky/illegal methods.