Block states include more information than just the block itself. Right now the code you have detects ALL types of planks.
In order to only get oak planks you need to check the block state's variant as well.
E.g.:
world.getBlockState(blockPos).getProperty(BlockLog.VARIANT) == BlockLog.EnumType.OAK
Note that this is not 100% accurate, I don't have my dev workspace up at the moment, but you should be able to use this to find the enum inside the BlockLog class