You can't.
You don't buy oak trees. You can buy oak logs, or oak planks. You can also buy oak seeds, which in this case are called "acorns". Prices in RuneScape change all the time, since they are mainly driven by supply and demand. You can find updated prices here: at the top of the main RuneScape page, there is a kind of menu; select "Community", then "Grand Exchange".
Some good places to cut oak trees in RuneScape for free to play players include the two locations East Draynor Village and South-West of Varrock West Bank. For pay to play players, a good spot to chop down oak trees is the Tree Gnome Stronghold. Oak tree chopping is not a good way to get money in RuneScape however, but is good for training Woodcutting levels.
Go to Falador park. Go to the East after that. Near the tree patch and the Saradomin statue you should see some with the name "Garden Supplier"
For one, you must be a member to have a home. To start making a home, go to the Construction guy in Falador or Varrock. To begin building, you need to make a room. Build the room, get the supplies to make something, and start building.
an oak tree much?
An acorn comes from an oak tree. Oak trees produce acorns, which are a type of nut that serves as a seed for the oak tree to reproduce.
you can't get free oak planks on runescape now stop being a noob and make them yourself. -.-
An oak tree.
Runescape garden patches, or common tree patches. But you can also plant marker plants everywhere.
The largest oak tree is the Dr. Oak from Pokemon.
They are purchased from the garden supplier in Falador park for 10,000 coins.