Getty ImagesBig oil companies are great dividend plays for retired income investors, if you know when to hold them.Large integrated oil companies can act as great dividend opportunities for retired income investors, the trick is knowing how and when to own them. Over the last several months crude oil has fallen nearly 20% to a spot price below $90 a barrel. Naturally this has put downward pressure on nearly every oil and gas name from equipment, to producers, explorers, and drillers. Gauging the energy complex using the SPDR Energy exchange-traded fund XLE, +1.86% the sector has given back all of its gains on the year and is roughly 15% from its most recent high. I believe this creates an excellent opportunity for investors with excess cash, or that want to rotate their stockholdings using a relative value perspective. Yet selecting the appropriate vehicle is a key component given there are many growth oriented ETFs that may not give you your desired results. One of my favorite ETFs for income investors is the iShares MSCI Global Energy Producers Index FILL, +1.18% due to its high exposure to megacap integrated oil producers globally.Names such as Exxon XOM, +1.05% Chevron CVX, +1.92%Royal Dutch Shell RDS.A, +1.86% BP BP, +1.47% and Conoco PhillipsCOP, +0.81% make up the top 10 holdings that account for over 52% of the fund. Furthermore, using a back of the napkin calculation of the funds most recent price to earnings statistic reported at the end of September puts current prices at roughly 16.5x earnings. That multiple is approximately 5 times lower than the current S&P 500 P/E ratio. In addition to pure valuations, the 30-day SEC yield on FILL is reported at 3.51%, which is right in line with many other broadly diversified high-dividend indexes. From a strategy perspective, investors should keep in mind that oil prices are likely to remain volatile, and with the current selloff under way, there’s no indication of a bottom in energy stock prices. However, looking out at the intermediate and long term, it's clear that the energy sector offers good relative value juxtaposed with other dividend paying areas contained within the S&P 500. From a trading perspective, an investor could choose to begin to make small purchases with the plan to add to their position if prices continue to erode.Using a dollar cost averaging strategy could help you attain the best cost basis possible without making a large bet all at once. Securing an attractive income stream in energy stocks now can also contribute greatly to your total return once the market begins to turn, since the most beaten down sectors stand the chance to gain the most when an uptrend re-emerges. As always, developing a plan and then implementing it decisively will always produce the best results. http://www.marketwatch.com/