The two fracking brothers: oil and natural gas. As brothers do, they’re always in a frenemy-ish competition, the ultimate competition being in price. While oil’s been top dog for a long time, oil is getting more scarce and natural gas more plentiful (hello, U.S. shale), closing the spread between the two commodities in recent years.
The oil to natural gas ratio is a relative measure of the price of natural gas and the price of oil. And note that, for half a century, natural gas had no real infrastructure (in the U.S., anyway) in which to be used. So oil companies literally burned it off as it came out of the ground, often along with their natural drilling for liquid oil.
Anyway, they are yin and yang, as natural gas actually grew to become a meaningful part of our energy pad. The changing prices reflect a change in demand for each fuel type, since higher demand for a scarce good means higher market prices.
Since oil price is in the numerator (on top) and the natural gas price is in the denominator (bottom), the ratio above one means higher oil prices...while less than one means higher natural gas prices.