Tia's behaviour is perfectly normal and is nothing to do with maternal instinct, or lack of it. If she chose to let Ozzy sleep next to her that would be just as normal
It's great that she's accepted him well enough to play with him, but it sounds like she needs opportunities to escape him too. If she's growling when Ozzy is in her face or trying to lay down next to her she's telling him that she's unhappy with his actions. If he ignores her and persists she may go a step further and bite him. So she could probably do with some help from you. There has to be a balance between letting them get used to each other and letting things get out of hand.
Do you ever walk or train them separately? Whenever I've had an older dog and a puppy I've walked them together once a day, then separately at least once a day too for the first few months. Walking Ozzy on his own will give Tia a break from him, will help him bond with you, help him not to be too dependant on Tia and will give you a chance to do some training with him.
Walking Tia on her own will give her some respite from Ozzy and will also help Ozzy to learn about being home alone.
Hopefully they may share a bed eventually, but it doesn't always happen. My 2 are much happier in their own beds or curled up on the sofa, one either side of me. They get on well, but they're not best buddies, and that's just how it is sometimes
