List<String> allObjectNameList = new List<String>{'account', 'case', 'opportunity', 'contracts', 'idea', 'product', 'order', 'solution', 'campaign'};
Set<String> allObjectNameSet = new Set<String>();
allObjectNameSet.addAll(allObjectNameList);

//String allObjectName = 'account,opportunity,case'; //Uncomment this line to test it by giving any 1st order value, and comment below line.
String allObjectName = Label.ObjectNamePrioritise; // Keep above line kind of value in your custom label. In same order will get in final OK list.
List<String> fixedOrderList = new List<String>();
List<String> remainingList = new List<String>();
List<String> finalOKlist = new List<String>();
if(allObjectName != null) {
    List<String>  objectNameList= allObjectName.split(',');

    for(String fixedName : objectNameList) {
        if(allObjectNameSet.contains(fixedName)) {
            fixedOrderList.add(fixedName);
        }
    }
    //Remove filtered fixed Name from All names list
    allObjectNameSet.removeAll(fixedOrderList);
    system.debug('----allObjectNameSet----' + allObjectNameSet);
	
    //Set is sorted here for remaining names so we have to reiterate on list and take value from there.
    if(allObjectNameSet.size() > 0) {
        for(String strTest : allObjectNameList) {
            if(allObjectNameSet.contains(strTest)){
                remainingList.add(strTest);
            }
        }
        remainingList.sort();
        finalOKlist.addAll(fixedOrderList);
        finalOKlist.addAll(remainingList);
	}
    else {
        finalOKlist.addAll(fixedOrderList);
    }
}
else {
    finalOKlist.addAll(allObjectNameList);
}
system.debug('----finalOKlist----' + finalOKlist);