This paper provides a full characterization for when the expansion of a complete o-minimal theory by a unary predicate that picks out a divisible dense and codense subgroup has a model companion. This result is motivated by criteria and questions introduced in the recent works concerning the existence of model companions, as well as preservation results for some neostability properties when passing to the model companion... The focus of this paper is establishing the companionability dividing line in the o-minimal setting because this allows us to provide a full and geometric characterization. Examples are included both in which the predicate is an additive subgroup, and where it is a multiplicative subgroup. The paper concludes with a brief discussion of neostability properties and examples that illustrate the lack of preservation (from the "base" o-minimal theory to the model companion of the expansion we define) for properties such as strong, NIP, and NTP$_2$, though there are also examples for which some or all three of those properties hold. (read more)