An Opportunistic Order is an opportunistic means of buying low or selling high during a given time frame. In other words, an Opportunistic Order attempts to "buy the dips or sell the rips" by a specific deadline. As the order nears the deadline, it becomes less and less opportunistic as its limit price approaches the current market price.
In the following example, the deadline is set for three months from October 1st. The orange, top line represents an Opportunistic "Sell" Order, and the aqua, bottom line represents an Opportunistic "Buy" Order. In the beginning, the limit price of the buy order is set to a low-ball price (stink bid). In this example, the low-ball limit is set to the current price minus two times the Average True Range (ATR). If that stink bid doesn't get filled, the trader resets the price each day so that the buy limit gets closer and closer to the market price. The gap between the current price and the limit price is narrowed each day by simply decreasing the ATR multiple (e.g. 1st 2.0, then 1.9, then 1.8... all the way to zero). The same logic holds true for the sell limit orders, except the starting limit is set to the current price plus 2xATR.
In this specific example, an Opportunistic Buy Order would have been filled on October 15th at $101.38; not exactly the absolute lowest price during this time frame, but pretty darn close! An Opportunistic Sell Order would have been filled on November 17th at $115.55. The peak during this time frame was 133.27, which was nearly $18 more than the filled price; not the greatest result, but in lieu of being able to predict the future, this result is not bad.