A little misleading here...there are 4 islands that cruise ships can go to...Kauai, Oahu, Maui and Hawaii. Most cruise lines stop at all 4. NCL starts and ends at Oahu, so, unless you arrive early, or stay beyond your cruise, you aren't "visiting Oahu" on your cruise. NCL doesn't stop at any more islands than most other cruise lines. Yes, occasionally, some cruises only stop at 3 islands.
Most cruise lines stop at one port on Hawaii, NCL stops at two.