Oil companies were extensively involved in alternative energy until sometime in the mid-1980s. Many oil companies had nuclear power divisions, and they were also the first major commercial purchasers of renewable power technology for use in remote and offshore production facilities. What if the oil companies had continued this trend and become major energy companies instead? Could they even get involved in the utility industry, investing in nuclear and renewable energy to help provide steady cash flow during downturns in the fossil fuels industry?
I don't think there should be any monopoly concerns for this given the fact that the companies are seeking to expand into electric power generation and won't be using their own fuels. There are some cases where utilities even own their own gas fields or coal mines that they use to power their facilities.
Oil companies might like to get into utilities too, because they have massive incomes and could lock in a guaranteed rate of return from electric sales. Electricity consumption and prices don't vary nearly as much as petroleum consumption and prices. A smart oil company could even invest in or push utilities during economic downturns, because it could probably get a good deal on material and labor costs. The lack of immediate demand wouldn't be an issue for nuclear reactors, which take years to build, or for renewable energy, which get subsidies and have priority for power sales.