OBS is my favourite store for cheap products. It is also very big and almost everything is there. When it comes to fruit I recommend going to a supermarket next door called Extra Stormyra, they have a more broad and in my experience fresh fruit.
The cheapest decor comes from Rusta, a Ikea like store with good basics and all of the essentials you will need for your stay. They sell products like pillows, plates, plastic boxes and hangers ets. Stores like Søstrene Grene and Flying Tiger have more modern and aesthetic decor. They sell products like candles, art supplies and school supplies ets. They are also quite a bit more expensive then stores like Rusta.
Are you looking for local art, places like Gundersons and Ingridoline are the prefect stores for that. They both sell unique designs inspired by Norway and its beauty.
If you are looking for a cheap thrift-store, then the red cross is a good shop. Be aware that there are people out there who have less money, so it is good to be respectful and not buy to much in those thrift-stores. There are also a hand of vintage stores in Bodø that have really fun and unique items. My personal favourites are Bodø vintage, Drabant vintage, Fretex Bodø and Jessen Brukt og Nytt.