Filtering a list by comparing enums against a user choice
When SelectedOppType == 5, all values in OpportunitiesList will be added to FilterdOppsList, but only after evaluating many conditions unnecessarily. It would be better to handle this as a special case,
and add all values of OpportunitiesList in FilterdOppsList before ever entering the foreach loop.

Next, in the foreach loop, since selectedOppType doesn't change throughout the loop, regardless of the value of opp, it would make more sense to use conditionals on the value of opp.OpportunityStatusID instead of selectedOppType. You can rearrange the conditions to switch statement.

Putting it all together:

if(selectedOppType == 5)
{
    FilterdOppsList.AddRange(OpportunitiesList);
}
else
{
    foreach (Opportunities opp in OpportunitiesList)
    {
        bool shouldAdd = false;
        switch (opp.OpportunityStatusID)
        {
            case OpportunityStatus.Active:
                 shouldAdd = selectedOppType == 1 || selectedOppType == 4;
                 break;
            case OpportunityStatus.Draft:
                 shouldAdd = selectedOppType == 2 || selectedOppType == 4;
                 break;
            case OpportunityStatus.Closed:
                 shouldAdd = selectedOppType == 3;
                 break;
        }
        if (shouldAdd)
        {
            FilterdOppsList.Add(opp);
        }
    }
}