I searched on MEG and came up with this:

Thread #1

Thread #2

Thread #3

From what I have gathered, they have 2 seperate switches. O/D on the shifter itself, and E/S (location?).

Seems like some of them either don't have overdrive, or don't have the OD OFF button...

Finally! Here's the info I was looking for. They do have 2 seperate buttons...

Goin' Round Traffic Circles @ 50Km/h!!! \m/ -- 1998 E0 SVT #2119 of 6535 \m/ -- 2003 Sentra SE-R Spec V