Countries don't need to accumulate USD to participate in the oil trade [1], only liquidity providers do. If you live in China, and want to buy oil to produce widgets sold in Europe, you hold RMB and need to accept payment in EUR. So you trade RMB for USD, buy your oil, later on trade widgets for EUR, and then trade EUR for RMB (if you want dividends) or USD (to reinvest). The person selling the oil is probably going to trade the USD in for Rial's, Pesos or Loonies.

(The market maker captures 0.0002 USD in compensation for the adverse selection risk he takes on.)

The particular dollars being used for these purchases tend to be reused. They flow from the market maker to China, China to Saudi Arabia, and Saudi Arabia back to the market maker. This facilitates the real trade (oil + french wine in exchange for widgets), but is merely a bookkeeping mechanism.

The only person who needs to hold USD is the market maker. They need to hold enough reserves so that normal variation (e.g., today 5 people buy USD, tomorrow only 3 sell) doesn't deplete their supply. I don't know that much about commodities markets, but this probably is a small fraction of all dollars out there.

None of this has anything to do with T-Bills. US bonds have low interest rates because the US government is believed to be highly unlikely to default.

[1] Many countries do accumulate dollars for other purposes. For example, they might accumulate a reserve of stable currencies as a hedge against hyperinflation. Or, as in the case of China, market manipulation.