If you look at the DS9 crew in season 1 Sisko is not a captain and is looking a bit washed up, Dax is a young lieutenant and Bashir kinda makes out that it was a surprise someone as talented as him would pick DS9.Yes, O'Brien had just worked on the transporter for four years on the Enterprise, and suddenly has taking care of everything on the entire station, and their technology isn't even Federation!
It seems to me that DS9 was the crappiest posting in starfleet so they were probably delighted anyone took the job.
Also O'Brien was very experienced and it is suggested that doing something as boring as the transporters was something he was over qualified but choose due to a battle weariness or some form of PTSD