I am also suffering from OSA and having symptoms like you and having an average sleep of 6hrs/day. But one thing I noticed is, when I get more than six hours of sleep, I would feel fresh and won't be sleepy throughout the day. I only have the chance to sleep longer than six hours on Saturdays, when I do not have to go to work. I work in a biotech company and when I am working with my computer, I would be very sleepy but when I am working in the lab I won't so, I guess the nature of your job contributes to your sleepiness. Obesity and suffering from diabetes also can be a cause for sleepiness and this is for sure.
If you can, try first sleeping for 7 hours (I do not have that luxury that's why I'm not too confident about this). Do some changes in your lifestyle one at a time if you are interested to know the cause. Check with me and I'll be happy to talk it out with you(email@example.com). Thanks.