In recent experiments trying different flavors with bourbon, I have found orange to be one of the most pleasant. The flavors seem to "synergize" (is that a verb?). Lemon doesn't work as well although the lemon-based whiskey sour, and its correlative the bourbon sour, is an acknowledged star in the constellation of cocktails. The flavor of orange is more subtle than that of lemon. Putting things another way, lemon works as well if you add more sugar than if using orange.
At first, I tried to dispense with sugar or other sweetenings when using orange. E.g., I'd add orange bitters only, or a dash of Charbay blood orange vodka, or a measure of fresh orange juice. All these imparted good orange flavor and melded well with the whiskey. But the citric addition upset the natural balance of the whiskey, its sweetness quotient that is.
By adding a touch of sugar or syrup to the orange-infused bourbon (or straight rye or a mixture of straight whiskeys and sometimes other whiskeys), this re-established the balance which the whiskey had its own. In effect I created an Old-Fashioned. But I did it from the ground up, sussing out its internal logic as it were.
Apricot works very well with bourbon; so does peach. Dark fruits do not, however. A blueberry Old-Fashioned? I don't think so... A possible, nay probable, exception is that prune-like flavors work well with rye whiskey or a whiskey in which the rye element dominates.