I think you're pushing too hard on this one. The timing is a little off, you're not sitting in the beat right or something, the the groove of your vocals is just a wee little bit off from the groove of the music..
It's like almost there but just off enough to notice it lol! Maybe relax and breath more, try to open up and relax your throat..
Maybe sing this a few times thru a monitor or with headphones and have his vocals in the mix, that may help you nail the timing..
I've seen Ozzy many times and he is always kinds chill with his vocals, it just kinda comes out for him..
Ozzy is like the Rainman of metal anyway, he is a tough style to nail.. You kinda gotta take his songs and make em your own when you do his arrangements,
Just my 2 cents, good job dude!
I can only sing when I am in my car or ****ty drunk..