Yes there are safe, but first make sure you have what you think is, second it has to be old stuff,not cut straight from the tree and put in tank, i had a massive branch in my tank which was four and a half feet in length and very think, looked nice, it came of my mums oak tree two years ago, we put it behind shed out of the way, i got a wire brush, removed all bark, scrubbed and sanded, then fill bath will baking hot water, scrubbed again and again, repeated about five times.
Then soaked it in the bath for a week, as all it would do was float,as i never use the bath it wasn't a problem,
Also a lot of channa breeder's, collect oak leaves, around this time of year, dry them out, then use them in the tanks to help lower and change water quality, a lot swear by this method,