What is the short cut on OS for spelling in which you type the first letter and below lists alphabetically more words; and with each new letter, the list of options shorten

F5 or Fn F5, if this box in unchecked in System Preferences > Keyboard:

When I got up this morning, I powered up the iMac running Mojave and the option+ESC worked for word completion in Apple applications. Later, when I signed into Big Sur 11.6.5, the option+ESC did not work, but shift+option+ESC worked for word completion in TextEdit — even though the Edit menu in TextEdit still shows option+ESC for completion. I could not find any shortcut that performed word completion in Pages v11.2. What a mess…

Does not work on an M1 mini running macOS 11.6.5, Apple extended keyboard, and with Pages v11.2, using either F5 or Fn+F5. Does work in TextEdit with either F5 key configuration. However, the OP's profile shows El Capitan…

The option+ESC (⌥⎋) (completion) shortcut with Apple applications.

I thought option+esc was the shortcut for Text To Speech?

I only checked while in Monterey. F5 or Fn F5 seems to work as noted by balsa2. At least in Monterey.

Doesn't work for me with Pages 11.2 in Monterey, only in TextEdit.