What is Ohiya?
Oftentimes in most Japanese restaurants, you will be served free water immediately after being seated without having to ask. It is chilled with ice.
The water may be self-serve at some restaurants, and others may even serve green tea or barley tea instead.
If you run out of water, you can also have it refilled. The water may be tap water or filtered water, depending on the restaurant. Incidentally, Japan’s tap water is safe to drink as-is!