If i am considering the 0-5 year irs spread for the USD market, would it be more accurate to use the fed funds rate or the OIS rate? I believe the OIS rate is calculated based on the fed funds rate, but am not sure how the dynamics of the relation work.
The OIS rate is simply the price of an interest rate swap where you're swapping the fed funds rate (paid daily) for a fixed rate. For example if a 3 month OIS pays 2%, and the fed funds rate is currently 0.5% then what we're saying is you can enter into a swap (at zero price) where you will receive 2% and pay fed funds. If the fed funds rate goes up you pay more, if it goes down you pay less.
The fed funds rate is quoted only as an overnight rate; if you're looking for a USD overnight rate you should use that. If you want a longer rate then you should look at OIS rates, which are quoted all the way out to 50 years (although I doubt there's much liquidity out there, it does appear on Bloomberg).