You are comparing an OIS Swap with a fixed float IR swap. In a fix float IR swap, at the time of swap inception, the fixed rate which makes the IR swap value as zero is the swap rate. In calculating the swap rate you need to discount the cash flows to inception date.
Usually the only payments in an OIS swap are done at maturity. In an OIS swap you don't need this discounting. You might as well say DiscountFactor * FixedLegRate = DiscountFactor * AveragedFloatLegRate. The AveragedFloatLegRate is not actually known until the end of the life of the OIS. It is estimated by the party entering in swap (just how the LIBOR float rate is not actually known and just estimated in a fixed float IR swap).
In your example, the party entering in the swap would have some expectation of the overnight rates for next 6 months. the geometric mean of these expected rates would lead to a rate=2%.
Note that discounting would be needed if the payments are annual (for longer dated swaps) or if there is a settlement delay after maturity.
The expression for AveragedFloatLegRate is given on page 200 of Anderson's Interest Rate Modelling: