So far I have tracked down 18 episodes Walter appeared in. I wonder if there are more.
http://fiveohomepage.com/walteromori/
He actually had dialog in two episodes -- Nine Dragons and Right Grave, Wrong Body -- though I dunno if that is really his voice or they dubbed someone else's later. I'm sure if it was really him, he would have been credited, especially in Nine Dragons, where his part is pretty long, though not as long as Murder With A Golden Touch. If you go to the pages for those two shows mentioned above, you will see the scenes where he has dialog.
I own the script for Nine Dragons and Walter's character's name in this show (not mentioned when it was broadcast) is Jimmy Tsen. The name of the other guy with him doing monkey business below the lab is Harry Luk. These two characters had additional action in the show, meeting Wo Fat when he arrived at the airport.
By the way, McGarrett's sexist line in this show to Professor Dalton -- "You're very pretty, and I'm sure you know a lot about chemistry" -- is not in the version of the script which I have, which is the "second draft" from April 9, 1976.