We purchased the LA Baby 3" Organic Cotton Compact Crib Mattress to go with our little one's ** ***** mini crib, also purchased on OS. This mattress fits perfectly with it and there were no gaps so that is definitely safe for our little one to sleep on. It has just the right firmness, not too soft and not too hard. We love the fact that it is made of organic cotton so this feature was a definite plus. Would recommend this product to other parents out there.