Lightly toast the sesame seeds in a heavy skillet for 5 mins (don't let them brown). Keep the pan moving to prevent burning.
Add the seeds to your food processor bowl, and whizz on high speed for 5 mins, until the oil starts to be released from the seeds. (Depending on your processor's speed, you may need to process a little longer. You could use a high speed blender instead.)
Drizzle in the olive oil, a little at a time, and keep processing until the tahini is at your desired consistency.
Store at room temperature in an airtight jar, away from direct sunlight. Some separation of tahini and oil may occur, just stir it in before using.