One of the earliest translations of Heidegger's works appeared in Walter Kaufmann's collection Existentialism From Dostoevsky to Sartre in 1956, the introduction added to the 5th printing (1949) of What is Metaphysics?, entitled The Way Back Into The Ground of Metaphysics. In it, Heidegger describes what he means by ontotheology.

Metaphysics states what beings are as beings. It offers a λόγος (statement) about the (ὄντα) (beings). The later title "ontology" chracterizes its nature, provided, of course, that we understand it in accordance with its true significance and not through its narrow scholastic meaning. Metaphysics moves in the sphere of the ὄν ῃ ὄν: it deals with beings as beings. In this manner, metaphysics always represents beings as such in their totality; it deals with the beingness of beings (the οὐσία of the ὄν). But metaphysics represents the beingness of beings [die Seiendheit des Seienden] in a twofold manner: in the first place, the totality of beings as such with an eye to their most universal traits (ὄν καθόλου κοινόν;) but at the same time also the totality of beings as such in the sense of the highest and therefore divine being (ὄν καθόλου, ἀκρότατον, θεῖον). In the metaphysics of Aristotle, the unconcealedness of beings as such has specifically developed in this twofold manner. (Cf. Met. Γ, Ε, Κ.)

Because metaphysics represents beings as beings, it is two-in-one, the truth of beings in their universality and in the highest being. According to its nature, it is at the same time ontology in the narrower sense and theology. This onto-theological nature of philosophy proper (πρώτη φιλοσοφια) is, no doubt, due to the way in which the ὄν opens up in it, namely as ὄν.

Pp. 217-218

So, if ontology is the study of beings, ontotheology is concerned with what makes beings possible, the ground or fundament of ontology--the open in which beings appear.  ¶ 6:37 AM