Honestly, ANY item is good in Pokemon Mystery Dungeon. Take it from someone who's been playing it for years. However, here's a few tips.
Scope Lens makes Critical Hits land more easily, which is nice, but I think you could do better with a different item.
- If your Oshawott partner has mostly/all Special Moves, give it Special Band.
- If it has mostly/all Physical moves, give it Power Band.
- If you don't like any of those items, give it Oran Berry for easy healing. If the IQ Skill Item Master is on, it might also throw the Oran Berry to heal you if you're in a tight spot.
- Personally, I like to change my item depending on the dungeon I'm going into. If I'm in a dungeon with lots of Sticky Traps, I hold No-Stick Cap. Carefully examine the mission you are going on and pick an item accordingly.