The contact light came on before touchdown. There were "feelers" hanging from the legs and these indicated sufficient proximity to the surface to shut down the engine. After "Contact light," there was a slight pause and Armstrong read the landing check list, starting with "OK, engine stop," then continuing, "Modes control both auto, descent engine command overide off, engine arm off, 413 is in." 413 was a code entered into the computer to tell it they were down.
So, I always say "OK" was the first word spoken from the surface of the moon, that is mechanically supported by it. This checklist is almost always elided when the landing is replayed, even in NASA material. Norman Mailer spelled it out in his book, OF A FIRE ON THE MOON, which is where I first knew of it, and it was on a National Geographic 45 rpm record inserted in a special issue of the magazine a year or so after the event. I still have that. I'm sure it only has so many plays in it, so I guess I should put it on my Ipod, or something.
But the fact is, the lunar surface contact sensing probes were attached to and part of the craft, which touched down within a matter of 2 seconds or so after contact light illuminated with the landing pad almost immediately contacting the surface. Technically you're probably correct with "OK, engine stop", being the first words. But what is known is the first word spoken from the surface was certainly not, "Houston" regardless of how much some want to be so.