Place the Oreos into the blender, and pulse a few times to break them into smaller pieces.
Add the cold milk and ice cubes to the blender with the crushed Oreos.
Blend on high for about 30-40 seconds until the mixture is smooth, thick, and has a creamy consistency with small crunchy bits of Oreo visible.
Taste the slushie and, if desired, add sweetened condensed milk for extra sweetness. Briefly blend again to incorporate.
Pour the thick, frosty mixture into glasses, allowing the crunchy Oreo bits to settle slightly at the top.
Serve immediately with a straw or spoon, enjoying the crunchy, chocolatey texture and icy refreshment.