Tempering the chocolate for the bark requires a double boiler. If you don't have a double boiler, improvise one by using a saucepan and a stainless steel bowl: Nestle the bowl into the saucepan, allowing a few inches of space to remain between the bottom of the bowl and the bottom of the saucepan. Keep an extra bowl of a similar size on hand to transfer the chocolate for cooling. Rest the chocolate thermometer on a towel. Line a baking sheet with foil.
Pour a couple inches of water into the bottom half of the double boiler and heat over medium-low to just below a simmer. Put the chocolate in the top half of the double boiler and set over the hot water. Slowly melt the chocolate, stirring with a heat-resistant rubber spatula. As the chocolate melts, check the temperature periodically to make sure it stays between 82 and 86 degrees F. If it starts to rise above this temperature, quickly transfer the chocolate to the bowl on reserve and stir briskly to reduce heat. Return bowl to double boiler to maintain heat between 82 and 86 degrees F.
Remove bowl from over the hot water just before all the chocolate melts. Stir vigorously until the chocolate melts completely. Check temperature again. Stir in the oil until evenly blended. Chocolate is now tempered and ready to use. Keep within the 82 to 86 degree F range, returning bowl briefly to the double boiler if necessary.
Quickly stir the fruit and nuts into the chocolate. Spread chocolate mixture out on the prepared baking sheet so it's about 1/2-inch thick and the fruit and nuts are coated. Set bark aside at room temperature to harden. Break into angled pieces. Store in an airtight container at room temperature.