Code:
<script type="text/javascript">
function setOptions(chosen) {
var izbor = document.form3.celik;
izbor.options.length = 0;
if (chosen == " ") {
  izbor.options[izbor.options.length] = new Option('choose',' ');
 
}
if (chosen == "okc") {
  izbor.options[izbor.options.length] = new Option('C0270','C0270');
  izbor.options[izbor.options.length] = new Option('C0370','C0370');
}
if (chosen == "pc") {
  izbor.options[izbor.options.length] = new Option('C1330','C1330');
  izbor.options[izbor.options.length] = new Option('C1530','C1530');

}
if (chosen == "cc") {
  izbor.options[izbor.options.length] = new Option('C1120','C1120');
  izbor.options[izbor.options.length] = new Option('C1220','C1220');

 }


}
</script>

<select name="vrsta" size="1"
onchange="setOptions(document.form3.vrsta.options
[document.form3.vrsta.selectedIndex].value);">
          <option value=" " selected="selected"> </option>
          <option value="okc">Opsti konstrukcijski celici</option>
          <option value="pc">Poboljsani celici</option>
          <option value="cc">Cementirani celici</option>
        </select>

<select name="celik" size="1">
        <option value=" " selected="selected">Izabrati prvo vrstu</option>
      </select>

<input type="button" value=" ok "  onclick="form3.ans15.value(document.form3.vrsta.options[document.form3.vrsta.selectedIndex].value);" />

<input type="type" "number" value="" name="ans15" size="9" />
<input type="type" "number" value="" name="ans14" size="9" />
I dont know how to write something in ans15 and ans14 onclick of button
Example...if user selects new Option('C0270','C0270') and clicks on button it should write something in ans15 and ans14
I tried
<input type="button" value=" ok " onclick="form3.ans15.value(document.form3.vrsta.options[document.form3.vrsta.selectedIndex].value);" />

but it is not good...