Can I copy my u-lock key? I have an OnGuard u-lock with only one key.  I contacted OnGuard about getting a backup, but it requires a serial number which was included with the original packaging.  The number is not on the lock or key itself.  

So now I'm wondering if some locksmiths can copy such keys.  

I realize this description is inadequate, but for what its worth, the shaft of the key is fairly rectangular.
Not sure what kind of key is used on your specific lock.  The only image I can find of a OnGuard key shows it to be a slotted key similar to recent automotive keys, so most locksmiths should have the equipment to duplicate it.  The only question is whether they can get the correct blank.

Locksmiths can also duplicate the barrel-style keys used on many older bike locks, if they can find the right blank.

But I see a cheap OnGuards available at Target for $19.95 (with others ranging up to $80), while the new key is likely to cost upwards of $10.  So it may be best to just buy another lock (which comes with at least two keys, sometimes four).