MASS-Diver:
Agreed, as has been repeated throughout this thread - it's always a very good idea to go over signals before you dive with someone new.
Actually it's a good idea to go through hand signals before every dive as part of the pre dive check...unless you dive with the same person all the time.
You'll find anyone who's taken any sort of overhead training knows the 'hold' signal. It's not that important a command in open water is it is when you are down in a rusty pile of metal and your buddy notices monofilament around your foot...
There are 3 command signals. The 3 command signals must be responded with the like signal. Those are 1) the Ok sign. 2) The hold sign [as discussed, a closed fist, pointing towards buddy], 3) the 'thumb', thumbs up to terminate the dive.
With a hold: Your buddy tells you to hold, you respond with the hold, and then you hold until your buddy comes back and gives you the ok... which you return.
Now I've actually had a situation where I had a buddy return an ok with an ok... when he was -not- ok. If you are having a problem, and your buddy gives you the ok sign.. that is the sign that they are currently paying attention to you and you can try to signal a problem that needs to be resolved. "You ok?"... "No.. I can't get this damn light on"... "click"... "Ok now?" "yea, ok".
This stuff also helps with clarity of other signals. There is the 'query' signal, which is a closed fist with the index finger up in a hook. it means what is coming is a question. It looks a little like an 'ok', it looks a little like a 'hold'. So if they respond to 'query' with 'ok' or 'hold', then you know that they don't know the next signal is a question.
Which can get ugly if you want to check with your buddy to see if they want to turn around or not. "Query" "Turn around?" is definately different then "Turn Around!".
But the basic gist is you and your buddies definately need to be on the same page with the important signals before a dive. Ok, hold, abort, turn, exit, ooa, query, entangled, silt, bubbles, etc...