You don't battle Prof. Oak.
You get it from Professor Oak after completing the Sinnoh Dex, getting the National Dex and going to Pal Park.
By completing the Sinnoh Pokedex. Then go talk to Professor Rowan, he should give you the National Dex.
In Pokemon Platinum to get the National Dex you will need to have seen all 210 Pokemon in the Sinnoh Dex then talk to Professor Rowan and then after a while Professor Oak will give you the National Dex!
hopefully check out the johto Pokemon on the sevii islands after getting national dex from professor oak.
First see all the Pokemon in The Sinnoh Dex. Then go to Sandgem Town and talk to Professor Rowan. Professor Oak will come and upgrade your Dex to National Dex.
complete the pokedex then go to professor rowan and he will give you the national dex
Battle before the national dex - 52 Battle after the national dex - 68
You have to get all 210 Pokemon in the sinnoh dex. Then go to Rowan's office and professor Oak will come in and give you the national dex.
fill up sinnoh Dex and go to the Professor and Oak will come and upgrade your Dex
After completing the sinnoh dex, go to Prof. Rowan and Oak will arrive.He will give you the National Dex.
You can get the national dex from the professor after you have seen every Pokemon in sinnoh.
you need to first complete your first dex then go to the computer and and select rowan`s pc.then you can go to his lab and professor oak will walk inside and he will give you your national dex. signed brannon203