Opening price

The first of the four values every candlestick reports, and the interval is whatever the chart's time frame says it is. On a listed market the official session open is a more formal thing than a chart's first tick: it is the price struck in the opening auction, where orders collected before the bell are matched at a single price. On a market that trades continuously there is no auction, and the open of one bar is simply the first trade after the previous bar ended.

Against the previous close, the opening price is where a market expresses everything that happened while it was shut. The distance between the two is a gap, and its defining property is that no price existed in between: an instruction resting inside that distance becomes executable at the open and is filled at the first available price, which is the open itself or something near it rather than the level named.

In continuously traded markets the daily open is a convention rather than a fact about the market. Foreign exchange runs through the week without stopping, so the point at which one daily bar ends and the next begins is set by a provider's server clock. Two providers using different cut off times produce genuinely different daily opens and closes for the same currency pair, and anything calculated from them, including gap statistics and opening range conventions, differs accordingly. Neither series is wrong, and comparing figures taken from both is the error.