if statement when value is nothing

Hi,

At the beginning of my code, I set a value to nothing like this:
Set Oppen =  Nothing

Later on down in the code, I have an if statement to check that value like this:

if Oppen <> Nothing then........

It does not work.  I've tried these with no luck:

if Oppen <> Nothing then
if Oppen <> Null then
if Oppen is not null then
if Oppen <> Empty then

Thanks for your help,
Joel