Theory is the cafe inside OMSI on the south side of the building past the rotating exhibit hall. It's only open around lunchtime, so check hours. Closed Mondays except when Portland Public Schools is on break. You do not need to pay admission to go in the cafe; just let the admissions checker know you are going to have lunch in the cafe. Parking is $5 around OMSI and there is some limited street parking in the area.
There are limited outlets on the hard walls next to the windows. Look for one by the by the fire oven tables. A wifi password is required and can be obtained from anyone working in the cafe.
The food is really great, beer and wine available. Salt and Straw ice cream, pizza, and a changing seasonal menu. Self service casual style. Fireplace and lots of open seating. Good natural light. Outdoor space overlooking the river and Tilikum Bridge. Music can be loud-ish.