Is there any OTC medications for insomnia?

Antihistamines. Antihistamines like Diphenhydramine (benadryl) or similar sedating antihistamines are helpful in causing drowsiness. Also, supplements like melatonin may be helpful in insomnia.
Some options. First off, try to find out the cause for the insomnia and treat it if possible. Best not to use any medications at all and treat the root of the problem. However, there are some otc options to help with sleep including: tylenol (acetaminophen) pm, Benadryl or other forms of antihistamines. Melatonin can also help resetting circadian rhythm.
Antihistamines. Be aware that antihistamines can build tolerance, which means that they will only make you sleepy for a few days to a week. Use them sparingly or they will quit working for you. If your problem is more chronic, you need to see a doctor.