The course is run over 4 days, 2 full days and two 1/2 days. Here on Koh Tao the first day of the Open Water course starts at 4pm in the afternoon so you just need to arrive on one of the ferries getting to Koh Tao before then (you can book in your transport to the island through 12Go.asia). The first day lasts around 3 hours and mainly consists of watching the introductory PADI videos to familiarise you with some of the basics of scuba diving.
The second day consists of some more theory and some practical sessions in the swimming pool.
Day three is when you get in the sea for your first two scuba dives from the beach.
On day four you'll complete dives 3 & 4 of the Open Water course from the dive boat, on which you'll have the option to be accommpanied by a videographer who will produce a video which you'll be able to watch in the dive school bar that evening after 6pm and have the opportunity to buy a copy if you wish. You'll finish up your dives and certification process by around lunchtime on the final day of the dive course and be a PADI Open Water scuba diver qualified to dive down to 18m anywhere around the world.
There is a medical questionnaire that you are required to complete before you start the course, normally you will complete it here when on the first day of the dive course, but it is worth checking before you come as if there is anything which looks as though it may be a problem then your doctor may be able to sign you off as being medically fit to dive, you can download it here PADI Medical questionnaire and take a copy along to your doctor to sign off for you.