But the word appeared at least once in English even before Haeckel was born. See an article by "A.C.C." with the title "Stray Thoughts on Language III" published in The Gentleman's Magazine for October 1829, where we read (Stray Thought number XXIII, on p. 317):
A table of the principal rivers in the world has lately been published, which the editor has disfigured with the ridiculous title of "Potamology." These pedantic names for the sciences are now more assiduously, and of course more annoyingly brought into use than ever. That clever publication, the Athenaeum, used to contain a weekly record of scientific facts, which were pompously parcelled out under the barbarous nicknames of "Orology, Ophiology," &c. because mountains and serpents were treated of, and it lately inserted an article headed with the strange title of "Oikology" (which if the term were at all allowable, should, according to all derivative analogy, be written Ecology.) If such words as these are to be reckoned English, Schrevelius and not Johnson will soon become the standard dictionary.This issue of The Gentleman's Magazine is available through Google Books. I can't locate the Athenaeum article, and so I don't know what meaning was given to oikology there. This example of ecology precedes the first OED citation by almost fifty years. I don't know the identity of "A.C.C."