You can return output.innerHTML

function sliderControl() {
    var slider = document.getElementById("slider");
    var output = document.getElementById("output");

    slider.oninput = function() {
        output.innerHTML = this.value;
    };
    output.innerHTML = slider.value;
    return output.innerHTML;
 }
const val = sliderControl();

function generatePassword(val) {}

You can add a new variable.

function sliderControl() {
var slider = document.getElementById("slider");
  var output = document.getElementById("output");
  output.innerHTML = slider.value;
  slider.oninput = function () {
    output.innerHTML = this.value;
    return output.innerHTML;
  };
}
var yourVariable = sliderControl();

function generatePassword(yourVariable) {
    // do the thing with yourVariable
}

You could use the output.innerHTML like you have set it.

function sliderControl() {
  let slider = document.getElementById("slider");
  let output = document.getElementById("output");
  output.innerHTML = slider.value;
  slider.oninput = function () {
    output.innerHTML = this.value;
  };
}
sliderControl();

function generatePassword() {
  let length = document.getElementById("output").innerHTML;
}

I would also recommend to use let instead of var inside closures and functions. As var might cause unexpected side effects, as it can change the value of a already declared variable of the same name outside the functions/closure.