Preheat your oven to 350 degrees F.
In a medium bowl, mix together the all-purpose flour and black currants. If the currants are sticking together, be sure to break them apart and completely coat them in flour. Set the mixture aside.
In a large bowl, cream together the butter and brown sugar for about 2-3 minutes, until light and fluffy. Add the eggs, lemon zest, vanilla extract, baking soda and salt; mix until thoroughly combined. Fold in the oats until completely combined. Next, fold in the flour and currant mixture and mix until there are no longer any specks of flour.
Using a 2-inch cookie scooper, drop rounds of cookie dough onto a parchment-lined baking sheet, spacing the cookies about 2-inches apart and flattening each cookie slightly. Transfer the baking sheet to the oven and bake for 10-12 minutes, until the cookies' edges are lightly golden brown. Place the cookies on a cooling rack and repeat the baking process until you've baked through all of the dough. Cookies are good for up to 5 days in an airtight container.