void _FillTree(OracleDataReader reader, TreeNodeCollection nodes, int parent)
{
	while(reader.GetInt32(0) == parent)
	{
		TreeNode n = nodes.Add(reader.GetString(3));
		int id = reader.GetInt32(2);
		if(!reader.Read()) break;
		if(reader.GetInt32(0) == id) _FillTree(reader, n.Nodes, id);
	}
}
void FillTree(OracleDataReader reader, TreeView tree)
{
	tree.Nodes.Clear();
	if(reader.Read()) _FillTree(reader, tree.Nodes, reader.GetInt32(0));
}

...

// использование
OracleDataReader r = cmd.ExecuteReader();
FillTree(r, treeView1);