my leaderboard stats won't save?

This will work.

Furthurmore, you shouldn't save as you load, because if datastore takes too long to respond, the player looses their stats, I would say that you should use an autosaver that saves every 30 seconds to 2 minutes.

Also, saving everytime a value is changed is ALSO a bad idea. What if someone keeps "oofing" / resetting, like 10 times a minute, this will result in dataloss.

USE AN AUTOSAVER AND GET RID OF QUICK SAVES

--Varibles
local DataStore = game:GetService("DataStoreService")
local DataStoreoofs = DataStore:GetDataStore("SaveSystem_oofs")
local DataStorerank = DataStore:GetDataStore("SaveSystem_rank")

game.Players.PlayerAdded:Connect(function(player)
    --Create Leaderstats
    local LeaderStats = Instance.new("Folder", player)
    LeaderStats.Name = "leaderstats"
    --Create Values
    local oofs = Instance.new("IntValue", LeaderStats)
    oofs.Name = "OOFS"
    local rank = Instance.new("IntValue", LeaderStats)
    rank.Name = "RANK"

    --Quick Saves
    oofs.Value = DataStoreoofs:GetAsync(player.UserId) or 0
    DataStoreoofs:SetAsync(player.UserId, oofs.Value)

    rank.Value = DataStorerank:GetAsync(player.UserId) or 0
   DataStorerank:SetAsync(player.UserId, rank.Value)

    --Values changed (save)
    oofs.Changed:Connect(function()
        DataStoreoofs:SetAsync(player.UserId, oofs.Value)
    end)

    rank.Changed:Connect(function()
        DataStorerank:SetAsync(player.UserId, rank.Value)
    end)
end)