My 6 month old baby has just started rolling onto her tummy through the nights to sleep but sleeps face down an dosnt turn her head to the sides, I keep putting her back onto her back but 5 mins after she rolls back onto her tummy. Should I be concerned that she isn't breathing properly or getting enough oxygen? I don't know what to do or if I should be concerned, im not really sleeping well because she is doing this. Shouldn't she be turning her head to breathe instead of laying face down?

Most babies will be rolling over by 4- 5 months…...some earlier, some later.                             
This is normal as babies grow & become stronger.
Many babies then prefer to sleep on their tummy or side.
This is not something that needs to be stopped. It is important to make her environment as safe as possible.

Is she sleeping bag one with her arms out?
She needs the space of the cot & arms free to roll well.
How long has she been rolling for?

Ensure that the cot mattress fits well & is firm & flat & there is nothing soft or loose in her cot, especially as she is face planting at times.
You can try to turn her face if you see her with face down, but often, like rolling to tummy, babies will just go back to their position of comfort.

How does she go with tummy time when awake?
**Increasing tummy time when awake is recommended to help build upper body strength so she can roll easily & well.
Always continue to place baby on back to sleep.

Red Nose has the following general information now that she is rolling . 

“Give baby extra tummy time to play when awake and supervised as this helps baby to develop stronger neck and upper body muscles which in turn enables them to roll back over.
Make sure that baby is on a firm and well-fitting mattress that is flat (not tilted or elevated).
Make sure that baby’s face and head remains uncovered (do not use lambswool, doonas, pillows, cot bumpers or soft toys)
As babies grow and develop they become very active and learn to roll around the cot.
At this time, continue to put them on the back at the start of sleep time, but let them find their own position of comfort.
By this stage it is not necessary to wake during the night to turn baby over to the back position

Do not use any devices designed to keep baby in a particular sleep position.”
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over