Here is a link I found about IAL..
Yes oak leaves release tannins into the water and it's better than nothing, I even have oak tree's and have used them , But as far as I know the chemmical properties of the tannin's in oak leave's has not been proven as much as IAL. And bettas dont live in oak leave water they live in IAL black water so even thoe oak release's a tannin's into the water it dont work as good as IAL . The oak tannin's is known to release minerals and lower PH, but has not been proven to make the fish scales tougher so there less likely to have any problem's or cut tare's..... and yes oak does make a tannin's, it's not the black water extract of an IAL that bettas live in.
The link I posted above is not the one where I seen pro's and con's of the IAL vs Oak. I will keep looking.
And im not saying dont use oak, I have even used oak bark. If you cant afford IAL and you have oak around you, then use it because it's better than nothing...