Fall makes Fools think of foliage...and stocks. Image: La Hestre (Belgium), the European beech of Mariemont. By Jean-Pol GRANDMONT via Wikimedia Commons

A disciplined strategy of regularly buying stock in outstanding businesses can help to compound an investor's wealth over time. In this way, even small sums can grow into a large fortune. With that in mind, we asked five of our top contributors to name the company they believe would make for an excellent long-term investment for an investor buying stock in October. Here's what they had to say about Apple, IBM, Celldex, Total, and ONEOK.

Jason Hall (ONEOK): The oil and gas industry has been rocked on its heels over the past year-plus, with oil and natural gas prices both plummeting:

WTI Crude Oil Spot Price Chart

WTI Crude Oil Spot Price data by YCharts

Investors have lost tens of billions of dollars since last year, selling out of oil and gas stocks to avoid greater losses as the downturn lags on. This has also created a fantastic opportunity for the contrarian investors out there: ONEOK (NYSE:OKE).

The thing is, ONEOK's main business, operating natural gas pipelines (held by its master limited partnership ONEOK Partners) is largely unaffected by NG prices, but instead driven by demand.

Demand for natural gas is strong, with U.S. production up about 50% over the past decade. Between increased use in domestic energy production, chemical manufacturing, and soon to be exports, demand is expected to continue to grow in coming years.

It's not a "risk-free" stock. ONEOK is facing some short-term pains in its natural gas liquids gathering business, where it does have commodity price exposure. But the market's beatdown looks to be far overdone, based on the level of potential short- and long-term harm from NGL prices.

If you're willing to stomach a little risk and the likelihood of more volatility with energy prices, there's massive upside, and a nearly 7% dividend yield on the table today. Now's a great time to take a long-term position in ONEOK.

Tyler Crowe (Total): Like Jason, I believe there are amazing long-term buying opportunities presently available in the energy sector, and the one that looks the most compelling today is Total (NYSE:TOT).

The reason that Total looks like a better buy than its other Big Oil peers is because it has been able to maintain the best margins on its oil and gas production during this downturn, and because it has one of the best long-term production growth strategies in the business right now.

Source: Total investor presentation

That 900,000 barrels per day in new production by 2018 not only looks good from a growth perspective, but also because the company is building these plans based on the assumption that oil will be in the $60 range for a very long time. This projection is much more conservative than all of its peers aside from Exxonmobil.

On top of it all. Total's shares are trading at historically low levels. Its price-to-tangible-book value ratio – a better valuation metric for cyclical businesses – hasn't been this low in over 20 years.

TOT Price to Tangible Book Value Chart

TOT Price to Tangible Book Value data by YCharts

A great company in the middle of a cyclical downturn trading at a once-in-a-generation valuation? Sign me up!

Todd Campbell (Celldex): Investors interested in owning a bit more risk in their portfolios might want to join me in searching for an entry into Celldex Therapeutics (NASDAQ:CLDX).

Celldex Therapeutics shares have been crushed since August for two reasons. First, the company dashed hopes that its promising brain cancer therapy Rintega could win over regulators without phase 3 data this summer and second, worries of price controls to rein in sky-high drug costs has led to a broad-based sell-off in biotech stocks that's taken Celldex Therapeutics' shares even lower.

Although both of these reasons shouldn't be ignored, I think that the selling in Celldex Therapeutics is overdone. Although Rintega may not make it in front of regulators until after phase 3 data is in hand, we're still looking at a potential FDA filing for approval in 2017 and that's not all that long to wait. Also, while drug prices are high, the focus on curbing prices isn't likely to center on stifling the development of breakthrough treatments addressing life-threatening diseases with a high unmet need, such as glioblastoma.

Obviously, I have no idea where shares are heading from here, but given that Celldex Therapeutics is likely to present additional Rintega trial data in November at an important industry conference and its shares have traded up in the past two years following that conference, I think October is as good a time as any to consider picking up some shares.

Tim Green (IBM): It may seem as though there's no urgency to buy shares of IBM (NYSE:IBM), given that the stock has been depressed for the better part of a year. IBM stock dipped into the $140s and stayed there during September, a level not seen since the end of 2010. For 13 quarters in a row, Big Blue has posted revenue declines, and the market is becoming increasingly pessimistic.

But the core of IBM has proved to be resilient. The mainframe, which directly and indirectly accounts for 35% of IBM's profits, according to an analyst from Sanford Bernstein, remains entrenched in industries ranging from banking to retail. Through decades of upheaval in the tech industry, the IBM mainframe has stood the test of time, and continues to be a cash cow for the company.

It will take more than the mainframe to drive growth, though, and some of IBM's various growth initiatives seem to be picking up steam. Watson, the cognitive computing system, is making headway in the healthcare industry, with numerous partnerships announced in the past month, including a pact with Teva Pharmaceutical. Meanwhile, a partnership with Johnson & Johnson, announced earlier this year, is close to paying off, with J&J planning to launch a virtual patient coaching app built on the Watson API.

With shares of IBM now trading at just about 9 times the low end of the company's EPS guidance for 2015, investors are getting an incredible deal, assuming that IBM can eventually turn things around. IBM's results will continue to be messy as the company's transformation continues, but long-term investors can take advantage of this uncertainly and buy a great company at a bargain price in October.

Source: Apple

Joe Tenebruso (Apple): Apple (NASDAQ:AAPL) has long benefited from the iPhone subsidies offered by major wireless carriers here in the U.S. By offering consumers the opportunity to purchase a new iPhone for an upfront cost of $200 (and a low as low as $0 down for older models), these subsidies lowered the psychological hurdle of buying the premium-priced iPhone, as much of the cost of the phone was somewhat hidden in the higher monthly fees the carriers would charge to recoup the cost of the subsidies. And by offering their customers the option to upgrade to a new iPhone at that subsidized price every 24 months, the major carriers helped to ensure that most iPhone users would remain on a two-year upgrade cycle -- far shorter than the 3- to 5-year replacement cycles typical for iPads and Macs, which are sold without subsidies.

So when the largest U.S. wireless carrier, Verizon,announced that it would no longer be offering subsidies on iPhones, many Apple watchers feared that the company's all-important iPhone business could take a hit. That's because while its new plans would have consumers absorb the full cost of their iPhones (over a two-year financing period), Verizon would help to offset that cost by lowering its monthly charges. The fear was that this would incentivize many consumers to wait longer to upgrade their iPhones, so as to enjoy the savings from their lower monthly charges for as long as possible.

However, Apple itself has entered the mix by offering its own iPhone financing plan, which gives users the ability to upgrade their phones to the latest model every year. All the major carriers have followed suit and now offer similar upgrade plans. If these plans prove popular among consumers, and I believe they will, they have the potential to accelerate the iPhone upgrade cycle. Such a scenario would see Apple enjoy more sales more quickly of its highest-margin product, which would likely be a boon for shareholders. Now looks like a good time to buy Apple stock.