The caramel cools quickly, making it impossible to roll all the hazelnuts before the candy hardens. Enlist the help of others, or make only half a batch, snacking on any that remain in clumps.
Lightly coat a nonstick baking mat or a piece of parchment with oil, and place on a heatproof surface. Bring sugar and water to a boil in a small saucepan over medium-high heat, tilting pan to swirl and washing down sides with a wet pastry brush to prevent crystals from forming. Boil, without stirring, until mixture turns pale amber, about 8 minutes.
Remove from heat, and stir in hazelnuts. Pour mixture onto center of prepared surface. Working quickly and using gloved hands lightly coated with oil, roll each nut between your palms until completely coated and smooth. Let cool completely. (If praline mixture begins to harden before you roll the hazelnuts, transfer the mat or parchment with the mixture to a baking sheet and place in a 375-degrees oven until pliable; you can do this only once before the sugar begins to crystallize.)
To determine if the mixture has reached the soft-ball stage the old-fashioned way, drop a little into cold water. If it forms a small ball, it's ready. Pralines can be stored in an airtight container at room temperature for up to 2 weeks.