thank you to @user42108 for the link to OpenGamma conventions PDF in his answer below. The PDF is comprehensive and explains the mechanics of USD OIS Swaps based on Effective Federal Funds Rate (EFFR) well, I paste an extract from the PDF here:
"Federal Fund swaps are a USD particularity. They are swaps exchanging quarterly USD Libor payment for quarterly average of USD-Effective Federal Funds Rate. They are often called the Feds or Fed swaps. The particularity is that the rate paid is the arithmetic average of the fed fund rates; the rates are not compounded like in the traditional OIS. The quarterly coupon payment is not equal to a three months OIS. The code on Bloomberg is USBGx Curncy with x the tenor"
As we know, USD Libor rate will be replaced by SOFR rate by the end of 2021, and my understanding is that there are already traded SOFR OIS Swaps: to encourage other answers, are there any knowledgeable users of the SOFR OIS Swaps? What is the exact mechanics of these? Is the liquidity of these comparable to the EFFR OIS Swaps? Do you think the market will eventually favour USD OIS swaps indexed to SOFR as opposed to USD OIS swaps indexed to EFFR?
Finally, notice the definition of the USD "OIS" swaps indexed to EFFR, cited above from the OpenGamma PDF (these EFFR swaps are indexed against USD Libor, so effectively are basis swaps, rather than classical OIS swaps that are vs. fixed): will the USD-Libor leg on these swaps be replaced by a SOFR leg? (So effectively, these swaps would become EFFR against SOFR: not sure that would make much sense as a product??)
(i) Are USD OIS Swaps indexed to SOFR (Secured Overnight Funding Rate), EFFR (Effective Federal Funds Rate) or the FFTR (Federal Funds Target Rate).
(ii) If USD OIS Swaps exist on multiple benchmarks, which ones are the most liquid ones?
(iii) What are the exact mechanics of these swaps? (For example, EONIA-indexed OIS swaps use a kind of backward-looking geometric average of realized overnight EONIA rates to compute the floating leg: how does the USD OIS mechanics work?)
(iv) Finally, are there any SOFR-EFFR or SOFR-FFTR basis products?