My diagnostic reader shows bank 1 is lean on my 1998 E150 van, what could this be?

Vacum leak, misfire, dirty air meter.

What is the code? Lean condition is too much air, like a vacuum leak, The pcm is trying to compensate by adding more fuel, causing you more grief.

O2 sensor possible

It is possible, but it would throw a code indicating a problem involving the 02 sensor. I wouldn't want to tell you to just replace it.