Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Koh Samui to Chicago as you normally would.
Located just 3.3 km from the city centre, Ko Samui is the airport most travellers will use when flying to Chicago from Koh Samui. Booking flights from Ko Samui (USM) to Chicago should be relatively easy as there are 0 daily outbound flights to Chicago from 0 different airlines.