There are a large range of hotels around Oslo at various price levels. Due to the public transport, it is relatively convenient regardless of which area you choose. The four closest hotels to the conference venue are:
Scandic Holberg (30 seconds from the venue)
Rassison Blue Scandinavia hotel (1 minute from the venue)
Thon Hotel Europe (2 minutes from the venue)
The closest budget (but very nice) accommodation to the venue is Cochs Pensjonat (10 minutes’ walk from the venue)