In input.conf, put something like this: `B script-message toggle_osc`

mpv_toggle_osc.lua

--[[
    Toggles OSC between "auto" and "always"
    https://gist.github.com/tunalad/d00a8e6ba5e2e2cd16619df0ea8e785d
]]--
local osc_visible = false

local function toggle_osc()
    osc_visible = not osc_visible
    if osc_visible then
        mp.commandv("script-message", "osc-visibility", "always")
        mp.osd_message("OSC: Always", 2)
    else
        mp.commandv("script-message", "osc-visibility", "auto")
        mp.osd_message("OSC: Auto", 2)
    end
end

local function on_key_press()
    toggle_osc()
end

mp.add_key_binding(nil, "toggle_osc", on_key_press)

local function observe_osc_visibility(_, value)
    if value ~= nil then
        osc_visible = (value == "always")
        mp.osd_message("OSC: " .. value, 2)
    end
end

mp.register_event("file-loaded", function()
    mp.observe_property("osc-visibility", "string", observe_osc_visibility)
end)