Very little DM work in Koh Tao... the place is an instructor factory...so an OWSI or MSDT can do the job, plus so much more. But some schools might keep you on after qualification IF they have need and IF you are exceptional (and IF you are willing to work for peanuts).
DM course in Koh Tao won't teach great dive skills. The island is synonymous with cheap, fun diving - not quality. That said, there's masses of dive students, lots of chance for assists and it's high volume tourism... so you learn how a busy dive operation works.
Most DM trainees are backpackers who go up-sold from Open Water to DM (or instructor)... it's a young crowd. Most of the instructors are interested in boosting their miserly commissions through encouraging continued-education and kit sales, getting laid and enjoying a beach lifestyle for a while. It's not the place for developing really high quality diving skills.
Koh Tao is an amazing place to stay and integrate with the dive industry. There will be plenty of nights bare-foot dancing at Lotus Bar or celebrating in Choppers... Try to be there for Song Kran. It's a good place to be single, also
The diving is a 'low-average' for SE Asia.... Chumporn Pinnacle, Twins and Sail Rock are ok.... the other sites are smashed bare by ill-trained open-water rototillers. The location is good for placid training conditions, but don't expect world-class by any stretch.