Can someone explain to me the rationale for why the market may be moving towards OIS discounting for fully collateralized derivatives?
Most counterparty agreements specify some sort of ois rate for the interest paid/received on posted collateral. So the OIS rate is the appropriate one to use for discounting future cash flows.
Prior to 2008 the OIS/Libor spread was small and stable, so you didn't really need to worry about this, but now it's much larger, so people are taking it into account. The reason it's "big news" now is that properly switching pricing systems over to use OIS discounting is a large change, so most places are only now getting this online.
The OIS rate is more stable than Libor, right? And according to this article from Risk Magazine:
If you assume that you do not have any market risk (a strange assumption, but it would hold for example if you are fully hedged), then a (correctly) collaterlized derivative does not have any net future cash flow. Clearly: if the derivative contract has a cash flow of -X, its value will go down by X and the collateral account will have a cash flow of +X (the corresponding collateral will be returned).
If there is no future cash flow, there is no discounting (in the sense of funding costs). However there is a new question now: what is the correct amount of collateral C we should post in t=0 to collaterlize the cash flow in t=T?
Since collateral is accrued according to the collateral contract by the OIS rate we would like to have that the accrued collateral account matches the cash-flow, that is
C * (1 + r * T) = X
where r is the OIS rate. That is we determine the collateral by OIS discounting
C = X / (1 + r * T).
OIS discounting is the way to determine the amount of collateral we have to post.
(You can make this argument mathematically correct (under some general assumptions) and show that collateralization is like having a different currency which has its own interest rate, I have some stuff on this (paper, spreadsheet for OIS bootstrapping, source code) here: http://www.finmath.net/spreadsheets/curvecalibration/ )
There are three interest rates in play here:
In order for the collateralisation process not to generate surplus value to either participant, considering a single cashflow received today and then repaid a day later will give the answer that we need 1 = 2 = 3. Otherwise there will be a net flow of cash on today or tomorrow (or both).
First, consider net flows today. The person paying for the cash flow is paying out market value & receiving the collateral call value. We need 1 = 2 for the two to balance.
Second, take net flows tomorrow. That same person will be receiving the cash flow and repaying collateral with interest. For the values of these to balance then we need 2 = 3.
So then 1 = 2 = 3 in our example. Discounting for market value must follow discounting at the rate determined in the collateral agreement.
And since you can essentially synethesize any derivative by pasting together 1-day cashflows of this nature then this also estabishes that 1 = 2 = 3 for all derivatives.