Why does my oil light come on when the motor gets warm?

You might want to make sure you are using a straight 30w, or heavier, oil and that the oil is not broken down by heat or contaminated by gas/solvent. If it is not the oil itself, then it could be the sensor (hopefully) or some bearing is getting loose enough to impact the oil pressure when everything's warmed up.

If you are using 20/50 and your oil level is within spec after it has been warmed up then you have an oil pressure issue. For a quick test, with the bike running, look into the oil tank. Do you see air bubbles and the oil moving? Another quick test: Place a pan under the oil filter > remove oil filter > start bike (it'll take 10 sec of run time). Do you see any oil being pumped out from the feed line into where the filter would be?