If your oil fill cap leaks, a new o-ring might fix it. I have some o-rings that size, and they are maybe 20% thicker than the o-ring on my filler cap. Maybe the engine heat causes the o-ring to deteriorate?
The Rebel 500 oil fill cap is five inches above the engine oil level – so it is not likely to leak unless the bike lays on its right side.
The right crankcase cover M6 bolts might be loose – there are 14 of them – the torque value is 12 N·m / 1.2 kgf·m / 9 ft lbs.
Rebel500 