I suppose you could argue that since the author has chosen to use a word like "ontológica", we should use "ontological" in English. I am suggesting a different approach, partly because I find it difficult to think of a suitable noun for "trastienda", and partly because "ontological ____" in this context really sounds a bit ridiculous in English, to be brutally frank. "Trastienda" presumably means what is behind the scenes, not immediately apparent. "Underlying" seems to me to convey that idea.

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Note added at 1 hr (2014-08-29 02:24:04 GMT)
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I don't think "trastienda" implies anything underhand here.

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Note added at 10 hrs (2014-08-29 11:59:09 GMT)
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You certainly could put "underlying ontology" if you think it's necessary to emulate the Spanish terminology. But I still think this would be a mistake.

Let's be clear about what "ontológica" actually means. Ontology is a philosophical term referring to the metaphysical study of the nature of being and existence. In Spanish, ontología is defined in the DRAE as "Parte de la metafísica que trata del ser en general y de sus propiedades trascendentales".

Perhaps this really is a metaphysical study of business clusters, but it seems very unlikely indeed to me. What seems very likely is that the author is using "ontológica" to mean "related to the being or nature of". In other words, he or she is doing what Spanish-speaking academic authors (and especially those trying to establish their intellectual credentials) are prone to do, and using a high-flown term to express a relatively simple idea. I would be prepared to bet that the thesis is actually about the nature of "aglomerados productivos" and how they really work. It is a business administration thesis, not a philosophy thesis.

Now in this respect, practice is different in English. English academic discourse is very jargon-ridden, but the jargon tends to be related to theories within the area of study in question. Unless the approach is genuinely metaphysical, a word like "ontology" or "ontological" would not be used in an English thesis title in this field, in my opinion. If I were the supervisor I would require it to be changed, not because it's pretentious but because it's (almost certainly) a misuse of the word.

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Note added at 11 hrs (2014-08-29 12:25:42 GMT)
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José's comment is interesting, however. "Ontological work" seems to me completely unviable, but perhaps it does refer to the entities involved in aglomerados. If so, you might translate it as "underlying entities". But unless we have access to the thesis itself we can't tell whether that is really what it means, and I am not very confident that it is.