I'm installing a new 1/2" white oak floor. I plan to sand it to 120 grit. What grit should I start at? In order to not take off too much material, my instinct is to begin at 80, then go to 100, then go to 120.

Is that ok or should I begin at a lower grit?

Also, should I use a drum sander or an orbital sander for this?

Thanks.

Depending on the material you can decide. Some flooring comes that is thickness sanded which is different than just coming through a molding machine, The abrasion thickness sanding will be a little more accurate to get a smooth floor. Some boards are much harder and will be slightly thicker. Usually starting with 40 grit will be the best for making the floor smooth because the over wood thickness needs to be cut smooth before changing to the medium grit paper. Using a drum sander will be the best to get the floor smooth. Then you can skip to 60 grit to almost completely remove the 40 grit scratches. A final sanding with 100 is what hardwood floor contractors use to prepare for finishing or staining. This should have removed all the previous scratches from the rough and medium grit papers.
If the floor comes with lots of over wood, 40 grit sanded diagonally first, then straightened off by going with the grain will save time. Diagonal sanding cuts faster than going with the grain and harder board will be smoothed with less effort.
Start your edging after the field is flat using 60 grit, then go to 100.