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TOCOM oil in USD 
TOCOM Crude is a cash-settled blend of Oman and Dubai crude oil, both quoted in USD.  The daily settlement price is mark to market, but the final settlement price is based on reported prices from Dubai and Oman (or calculated in some cases with a known procedure), averaged and then converted to Yen using monthly average exchange rates as published by a reference bank (see Detailed Rules) You're trying to go all the way back and unfuddle quotes into a blend of USD-quoted oils.  The correct procedure here would be to go with the Oman and Dubai prices in the first place (unless you're trying to arbitrage the TOCOM market). As to why they do it this way?  It's a service.  TOCOM takes on all the challenges to provide customers with a steady and consistent way of trading cash-oil.  For physical oil, all you'd have to do is buy the blend on Dubai's and Oman's spot market.  You trust TOCOM's price finding process, i.e. there will be no discrepancies between your TOCOM cash-oil and the Middle East physical oil. Edit: As to why Japan isn't buying WTI directly:  There's a considerable cost of carry.  WTI delivery location is Cushing, OK; there are pipelines but it's still a logistics act to get the oil to a port on the West Coast and then have it shipped to Japan.  Dubai's delivery is at Jebel Ali (Persian Gulf), Omani crude can be shipped straight from Mina Al Fahal.  Not only is it a shorter trip but also there are more shipping companies specialised in oil deliveries to the Asian hotspots. Why they pay in USD?  Persian oil is highly sought after in nearly all of Asia's economies but there's little other exported goods from there.  So naturally the market for currency crosses (AEDJPY, OMRJPY, AEDINR, OMRINR, etc.) is not that liquid.  At least not as liquid as to make buying Persian oil a smooth deal.  Anyway, both Dubai and Oman chose to follow Western practice to quote their contracts in USD and (maybe because of liquidity concerns) also to accept USD for payment only.