The biomimetic active ingredient hydroxyapatite is used in various fields of oral care [7,14,15,16,18,103]. It remineralizes early caries lesions [104,105,106,107], reduces the initial bacterial attachment to enamel similar to 0.2% chlorhexidine [19], and acts as buffer and a calcium and phosphate reservoir in biofilms [108]. To extend these preventive effects, hydroxyapatite can be used together with lactoferrin, xylitol, zinc, allantoin, bisabolol, and/or hyaluronic acid to achieve an antibacterial effect and to prevent/reduce gingivitis. In general, these combinations can be used in different oral care products (toothpaste, mouthwash, oral gel, etc.). However, each product has to be developed individually to avoid unintended interactions of the actives with other components of the formulation. Moreover, the stability, solubility, sensory properties, etc., of the actives in each product are important as well. The results of our search show that zinc salts have been described in more studies than the other analyzed active ingredients. A limiting factor in using, e.g., lactoferrin and hyaluronic acid in oral care products might be the relatively high price compared to “standard” ingredients of oral care products. Lactoferrin, zinc, allantoin, bisabolol, and hyaluronic acid are mainly used in oral care products for the prevention/treatment of periodontal diseases but also for other purposes (see above). Xylitol is mainly used to support caries prevention [46]. Xylitol is also used in chewing gums. It is important to mention that potential adjunct ingredients to hydroxyapatite are not limited to the ones discussed in this review because there are many other natural or natural-inspired active ingredients that can be used for various purposes [101,102]. A main goal in using such hydroxyapatite and adjunct ingredient(s) combinations in oral care is to provide safe and efficient alternatives to ingredients with potential side effects, such as tooth staining (chlorhexidine [109], stannous chloride/fluoride [110]), the risk of dental fluorosis in children (fluoride) [111], the potential risk of bacterial resistances (chlorhexidine) [112], or cytotoxic effects (cocamidopropyl betaine, sodium lauryl sulfate, and fluoride) [113]. Interestingly, the preventive spectrum of hydroxyapatite can be extended by the described adjunct ingredients (e.g., wound healing etc.; see above). Thus, various patient groups may benefit, for example, patients with periodontal diseases and patients with xerostomia.This review has some limitations, which will be discussed hereinafter. It focuses on the search of studies analyzing the selected adjunct ingredients, and most tested formulations do not contain hydroxyapatite. There are, however, some studies analyzing formulations containing hydroxyapatite combinations (e.g., hydroxyapatite with lactoferrin [114], hydroxyapatite with xylitol [115,116,117], and hydroxyapatite with zinc (with zinc included in the apatite lattice [55] and zinc salts as an adjunct additive [8,116])). For example, Nocerino et al. studied the effect of hydroxyapatite particles functionalized with lactoferrin. They found the combination of hydroxyapatite and lactoferrin to be effective against different bacterial strains (gram-positive and gram-negative) but also to be more anti-inflammatory compared to lactoferrin alone [114]. Consequently, future studies should analyze potential synergistic effects of combined hydroxyapatite and known or novel adjunct ingredients. Another limitation is that many found that studies tested not only the efficiency of the sole ingredient but the effect of a whole formulation, e.g., hyaluronic acid with chlorhexidine [100], or lactoferrin with other salivary enzymes [40]. Note that hydroxyapatite itself has also been tested without any other ingredients, e.g., by Cieplik et al., and hydroxyapatite was shown to be a calcium and phosphate source and an acid buffer in bacterial biofilms in vitro [108]. Fabritius-Vilpoux et al. have shown the formation of mineral–mineral bridges between hydroxyapatite particles and enamel surfaces in vitro [11,118], and Kensche et al. studied the reduction of the initial bacterial colonization by hydroxyapatite in situ [19].