Common words:
* Oder: Or
* Ohne: Without
* Okay: Okay (loanword)
* Oma: Grandma
* Opa: Grandpa
* Ob: Whether, if
* Obst: Fruit
* Oft: Often
* Oh: Oh!
* Öl: Oil
Less common, but useful words:
* Oberfläche: Surface
* Oberst: Colonel
* Oktober: October
* Öffentlich: Public
* Öffnen: To open
* Ort: Place, location
* Orange: Orange (fruit and color)
* Ostern: Easter
More specialized vocabulary:
* Operation: Operation
* Optimierung: Optimization
* Opposition: Opposition
* Orient: Orient
* Original: Original
This is not an exhaustive list, but it provides a good starting point with a variety of word types and frequency levels.