Is the open price of a security in a given time period simply the first price for that security in that period? I was reasoning on how prices for candles (especially the open price for each candle) are identified from a security's stream of quotes.
"Money talks; bullshit walks." This works for markets/trading, too Prices -- especially in a thinly traded market -- can be all over the place -- of which 90% might be bullshit. ("Ooo! Maybe somebody will hit "MKT" and I'll get hit on their mistake!" sort of things. Phooey.) So, if that's the case, what would an Open or a Close (or..... yes, a High or a Low) be??? It's not the quote, but the actual trade -- otherwise, it'd all be bullshit -- it'd be all hither-and-yon, and just not usable, *nor* something you'd care to record, study, form indicators from, or rely upon to make any sort of market decision. In a phrase, then, the answer to your question is that each of O-H-L-C are actual trades.
But if it is only a matter of a first trade closing, couldn't this be easily used to move price lower or higher to someone's convenience?
What are you asking? Yes, you can and will move the price, even if you don't intend to. Depending on what you're doing, it can be considered manipulation.
I was asking confirmation that a first large enough trade defines the opening price and that it seems quite easy to manipulate the market by closing a first trade on the price you desire.
Sure it's easy -- you go to the other side. But under no circumstances is that market manipulation -- that's "a market."