Can I use this with a Niacinamide serum? (The Ordinary)

Hypothetically yes. Depending on what other products you use (don’t over exfoliate by having too many actives in all the steps of your skin care) but if you have a basic cleaner that doesn’t have niacinamide the one from the ordinary should be ok. There are actives in this moisturizer so check the ingredients of your other products. The sweet spot for niacinamide is about 5% so maybe reduce frequency of niacinamide down a bit or dilute.