Should all button text start with a verb?

I'm reading The Grammar of Interactivity on UXBooth and finds it quite interesting. However, the article quotes David Hamill who said (or wrote):

Buttons are for actions, like “Get a quote,” “Download,” “Open an account,” “Go to checkout.” The text on the button should begin with a verb. Otherwise it’s not a call-to-action, just a button with some text on it. “More information” for example, is not a call-to-action.

But that would mean that the adjective "OK" as button text is wrong. If it's wrong - what should we use instead of OK? I'm confused, so that's why I'm asking "Should all button text start with a verb?"

Answer

The "OK" text of a button normally refers to an action mentioned above in the form, that will usually be a dialog.
For example a "Delete this thing" dialog's "OK" button means delete it, a verb.
Actually, I prefer when the verb is in the button, no matter if it's repeated, because a fast user can check the action to take without needing to read the dialog content.
In the example above the title would be "Delete this thing?" and the buttons "delete it" and "keep it".

So yes, buttons should be or refer to verbs.
There might be exceptions, hence should instead of must.