The interest rate varies based on collateral: https://oasis.app/borrow/markets. Eth is between 3-9% depending on liquidation ratios, and wBTC is at around 4.5% and both of these numbers are drops in the bucket when you consider that the collateral has appreciated a lot more than that.

And one more thing, it's not technically an interest rate, it's a stability fee (which btw, gets burned, used to go back to stakers, but they removed that).