To obtain an oak sapling, you need to surround any log with four castle rune bricks, place a thorn rose on the log, then right click it with a gaia spirit.
Turn your island into a beautiful green paradise
The seed for the world you see in the title screen is -3104067206791784920. Other seeds I've found that have a cool-looking island: 7550444640844751794, 276527397047639334, and -4419318842425835617