For each business day in the period, you multiply together 1 + rate × the days the rate applies to ÷ 365, subtract 1, and multiply by 365 ÷ the number of days in the period to annualize the result. For non-business days, the preceding business day's rate is used for the number of days until the next business day.
The rate is not set until the rate for the last business day of the period is known. In a Japanese yen OIS, the floating amount is calculated on the end date of the period, after the Bank of Japan publishes TONA for the preceding business day.