In a large skillet over medium high heat, add turkey legs and olive oil. Brown turkey legs for 2 minutes per side, then transfer to the slow cooker.
Add all remaining ingredients to the slow cooker. Cover and simmer for 5-6 hours on high setting or 10-12 hours on low setting until the turkey leg meat is falling off the bones.
Strain stock through fine-mesh strainer into a large bowl or pot. Do NOT discard solids yet. Transfer solids to a separate large bowl or pot, let cool slightly for about 15 minutes, then pick out the turkey meat and save for another turkey dish such as Turkey Tetrazzini or Turkey Tacos. Discard remaining solids and bones.
Let turkey stock cool to room temperature for about 1 ½ hours before refrigerating. (Stock can be refrigerated for up to 2 days or frozen for up to 4 months.)