Oil prices have been on fire over the past year, rising more than 20%, which pushed the U.S. oil benchmark (WTI) up to a recent price of around $67.50 a barrel. With continued cooperation by OPEC, healthy demand growth, and a hesitant approach by shale drillers, WTI could soon top $70 a barrel. That's well above the $50 a barrel most producers in the country budgeted for in 2018.

Because of that, U.S. oil companies should produce a gusher of cash flow this year. While some oil stocks reflect this upcoming windfall, others haven't caught up, making them screaming bargains right now. Four that stand out are Devon Energy (NYSE:DVN), Marathon Oil (NYSE:MRO), Apache (NASDAQ:APA), and Anadarko Petroleum (NYSE:APC). Each one currently sells for less than 10 times trailing 12-month (TTM) cash flow from operations (CFO) per share while most rivals fetch a mid-teens multiple.

Several oil pumps in a row at dusk.

Image source: Getty Images.

Still cheap despite the run-up

Anadarko Petroleum is a firm believer that its stock price is dirt cheap. That's why the company announced plans to buy back $2.5 billion in stock last fall, which at the time would have reduced the share count by 10%. That buyback has already driven Anadarko's shares up 43% since it started. However, even with that rebound, the stock still only trades for about 8.7 times TTM CFO per share.

That discount is why Anadarko recently added another $500 million to its buyback program, fueled by the fact that oil is well above its $50 budget level. In fact, at $60 Anadarko said it would generate more than a $1 billion in free cash flow this year, implying it will produce even more at $70 a barrel. That windfall would give the company a bounty to buy back shares, which could help push its valuation closer to the peer group average.

Plenty of fuel left in the tank

Shares of Marathon Oil are up nearly 15% over the past year thanks to higher oil prices and strong quarterly results. However, even with that rally, shares still trade at only 7.1 times TTM CFO per share. That's despite the fact that Marathon has excellent leverage to the price of crude, with its CEO noting on last quarter's conference call that a $10 improvement from $50 to $60 positions it to produce $500 million in incremental operating cash flow above its budget this year. As such, it would generate even more money as oil heads to $70.

The company currently has multiple options for that excess cash including further balance sheet improvements, acquisitions, and returning more money to shareholders. That last option seems increasingly likely given how cheap shares are since a buyback could help chip away at the discount.

A pipeline and an oil pump at sunset.

Image source: Getty Images.

Waiting for the growth engine to kick in

Shares of Apache have tumbled more than 20% over the past year even though crude has rebounded sharply. Because of that, the stock trades at about 6.6 times TTM CFO per share.

The reason Apache's shares are so cheap is that unlike most peers, the company needs higher oil prices to balance planned spending with cash flow since it's investing billions to develop its Alpine High discovery. The company has taken a methodical approach to maximize the value of that find, which slowed it down. However, Apache is getting ready to hit the gas, which when combined with other catalysts and its dirt cheap stock, makes it a compelling oil stock to consider buying right now.

Insanely inexpensive

Devon Energy is the cheapest in this group at just 6.1 times TTM CFO per share. Like Apache, shares of Devon have only gotten cheaper in the past year after selling off nearly 18% following a disappointing end to 2017.

However, Devon Energy quickly fixed the problems causing it to fall short at the end of last year, positioning it for fast-paced growth in 2018. Further, the company recently announced a $1 billion share buyback program that will enable it to start gobbling up its cheap stock. Meanwhile, it's working to sell off more non-core assets to unlock value and potentially buy back more shares. Given the wonders that buybacks from Anadarko and others have done for their shares, Devon's stock might not stay this cheap for much longer.

Cheap oil

This four oil stocks all trade at ridiculously low valuations given where crude prices are these days. Because of that, they have ample upside, especially as oil heads toward $70 a barrel, which would provide them with even more cash flow. It's money that most of these companies will likely use to buy back their cheap shares, which could fuel big-time gains for investors in the coming year.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.