FOSC is the oscillator frequency, as "OSC" means oscillator. So, it's 16 MHz. And TOSC is the oscillator period, which is 0.0625 us.
But, for the program execution, the clock source is divided by 4 to generate 4 phases, which are named as Q1, Q2, Q3 and Q4. And instructions are executed in 4 phases, so TCY = 4 * TOSC. So, each instruction is executed in 0.25 us, except the ones that require 2 TCY.
But the formula for the calculation of the TAD in the datasheet uses TOSC, not TCY.