Preheat oven to 200 degrees celsius (180 if using fan-forced).
Grease a large baking dish with butter and set aside.
Place the water, caster sugar, vanilla bean and cinnamon into a large saucepan. Bring to a simmer and stir until the sugar dissolves.
Add the peaches and cover with a lid. Simmer for 15-20 minutes or until tender. Remove the set aside until cool.
Peel the peach skins and discard them. Chop the peaches into large chunks and place in the prepared baking dish. Set aside until needed.
To make the crumble, place the oats, brown sugar, plain flour and butter into a bowl. Use your fingers to squeeze and break up the lumps of butter until crumbs have formed and all of the oats and flour are completely combined with the butter.
Sprinkle the crumble over the peaches and place into the preheated oven.
Cook for 35-40 minutes or until the crumble is golden and crunchy.
Serve with ice-cream or cream.