Preheat your oven to 350°F (180°C) and prepare a baking sheet with non-stick spray or parchment paper.
In a large bowl, cream the butter, brown sugar, and granulated sugar together until light and fluffy.
Add the eggs and vanilla extract, and mix until well combined.
In a separate bowl, whisk together the flour, baking soda, salt, cinnamon, nutmeg, and allspice.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Stir in the rolled oats and raisins (if using).
Scoop tablespoon-sized balls of dough onto the prepared baking sheet, spacing them about 2 inches apart.
Bake for 12-15 minutes, or until the edges are lightly golden brown.
Remove from the oven and let cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.
To make the filling, beat the butter in a large bowl until smooth and creamy.
Add the powdered sugar and vanilla extract, and mix until well combined.
If the mixture is too thick, add non-dairy milk one tablespoon at a time until it reaches a spreadable consistency.
Asemble the oatmeal cream pies: once the cookies are completely cooled, spread a generous amount of the filling onto the bottom of one cookie.
Top with another cookie, bottom side down, and gently press down to sandwich the filling between the two cookies.
Repeat with the remaining cookies and filling.