I think it's mainly because a high oil price is good for the economies of oil-exporting countries, like Russia, Canada and Norway (to name a few). More exports = more economic growth. More economic growth (all things equal) should equal higher interest rates which leads to a more valuable currency.

> More economic growth (all things equal) should equal higher interest rates which leads to a more valuable currency.

In fact it works the other way - if you have high interest rates - it means people expect your currency to lose value. It's one of the less intuitive facts about currency, and the reason I'm happy I took economy course even though I dropped out of it on 4th year :)

The reason is - in "perfect knowledge world" it shouldn't matter if you keep your money for a year on bank account in country X, or if you instead change them into currency Y today, and keep it in country Y for a year (different interest rates), and then change them back to currency X.

If these 2 scenarios produce different outcomes - it means one of the banks is paying people more than it has to - contradiction in terms if I've ever seen one :)

So:

   moneyInX * (1+interestRateX) = moneyInX * exchangeRateXToYToday * (1+interestRateY) / (exchangeRateXToYAfterAYear)

   exchangeRateXToYAfterAYear = exchangeRateXToYToday*(1+interestRateY)/(1+interestRateX)

In real world people don't predict correctly 100% of the time, but in stable einvironment it's close (it's modified by the risk factor).

BTW that's what's happening in Russia now - high interest rates, recession, and currency losing value.

EDIT: but you're right that it could be caused by low growth, just the low-high interest rates mediation isn't there.