January was a disappointing month for most investors as the ALL ORDINARIES (Index: ^AXAO) (ASX:XAO) plunged nearly 6% for the month on the back of falling oil prices and continuing concerns about global economic growth especially in China.

As would be expected, only a small selection of shares managed to deliver significant gains for the month with the majority of stocks finishing the month deep in the red.

Of the stocks that managed to post a gain for the month, perhaps the most surprising was marine services provider MMA Offshore Ltd (ASX: MRM). The company posted gains of around 30% for the month of January despite the continued fall in the oil price and lack of any significant new contract announcements. To put this in perspective however, the shares are still down by around 92% from their all time high of $4.25 reached in February 2013.

It would appear there may be some investors who believe the worst of the oil price decline is over and MMA Offshore will begin to benefit from new activity once the oil price begins to recover. Other investors may think the shares are oversold and be looking for a bargain considering the shares are trading at a significant discount to MMA’s book value of $2.10 per share.

Despite MMA Offshore’s recent share price surge, the shares remain a very high risk / high reward proposition. Sure, the shares still look extremely cheap compared to MMA’s book value, but until there is a significant and sustained rebound in the oil price, exploration and drilling activity is likely to remain subdued and many of the company’s assets may remain under utilised. In addition to this, the company is carrying around $440 million in debt that needs to be serviced and this increases the risk profile of the company in a time where earnings visibility is poor.

Another stock that surged in January was retailer JB Hi-Fi Limited (ASX: JBH). Its share price gained around 20% as investors piled into the stock following the collapse of its one-time rival in Dick Smith Holdings Ltd (ASX: DSH).

Some analysts believe JB Hi-Fi could stand to benefit from an increase of around $200 million in sales over the coming year that could translate into an additional $20 million in earnings. In addition to this, margins could also improve as a result of less competition and this could add another $20 million in earnings.

While it is too early to tell whether either of these scenarios will eventuate, investors have been happily buying the shares on the expectation this will occur. As a result, I believe much of the benefit from the potential profit gains appears to have already been priced in and the shares look to be fully valued at the moment.

Rounding out my three stocks that surged in January is private health insurance giant Medibank Private Ltd (ASX: MPL). The company announced a surprise earnings upgrade late in the month that resulted in the share price gaining nearly 17% to finish the month at $2.51.

The insurer surprised the market by increasing its earnings guidance by 27% and now expects operating profit of at least $470 million for the full year. Much of this upside is the result of reducing erroneous claims, improved hospital contracting and a slowdown in the growth of hospital utilisation rates.

Despite the positive upgrade from the company, the announcement also highlighted the future challenges private health insurers face with the rising costs of healthcare. Medibank is also facing challenges from slowing premium revenue growth as a result of fierce competition and now expects premium revenue growth of between 4.5%-5.0%, down from 5.5%.

Considering the challenges Medibank faces over the medium term, I find it difficult to justify the company’s current multiple of nearly 20x and, as a result, I would not buy Medibank at today’s prices.

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Motley Fool contributor Christopher Georges has no position in any stocks mentioned. Unless otherwise noted, the author does not have a position in any stocks mentioned by the author in the comments below. The Motley Fool Australia has no position in any of the stocks mentioned. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy. This article contains general investment advice only (under AFSL 400691). Authorised by Bruce Jackson.