If you’re using DAILY pivots, then yesterday’s O, H, L, and C are used to calculate the pivot levels for today.
You have to decide (or let your charting package decide for you) when a forex “day” begins and ends. The School lesson that you referred to says most traders use 4pm New York time. I disagree. I think it should be 5pm New York time.
Also, notice that several different formula sets are available for calculating pivots. The most popular pivots are the so-called Classic Pivots. The formulas used to calculate Classic Pivots do not use the Open; only the High, Low, and Close of the previous period are used to calculate pivots for the current period.
A chart can be divided into equal periods of any length — 1 hour, 4 hour, 8 hour, 1 day, 1 week, etc. — for use in calculating pivots. The general rule is that the previous period’s OHLC determine the pivot levels for the current period.
Below is a chart showing Daily pivots for the GBP/USD, based on 5pm New York time as the start of each day.
The Classic Pivots shown here were plotted automatically by my charting package, based on the previous day’s H, L, and C. I have marked the previous day’s Open simply for reference; it did not figure into the calculation of the pivots shown.
I added the day separators (green vertical lines) to illustrate the daily time periods.