OALD's entry seems strange considering that its big brother, the OED only has /ɛnˈdɛvər/ which is nevertheless still odd.
The sounds /ɨ/ and /ə/ (illusion and allusion or roses and Rosa's) are, as Mary correctly stated, very similar in unstressed syllables and therefore merged in many varieties, and there is a lot of variation as to which class different words belong to in unmerged varieties. I don't know of any dictionary that reasonably respects actual usage wrt. this difference.
I don't trust Merriam-Webster, which generally has the most reliable (AmE) transcriptions, here at all. It only records the /ɨ/ (illusion)-variant for most words with inital unstressed syllable ("below", "behind", "beneath", "deceive", "excuse", "endeavor" etc.) where /ə/ is much more common in my experience in actual speech, but at least they get "believe" right and record both variants for "enclose".
I'm somewhat surprised about hm's comment about "-ed" suffixes which I have always thought is more commonly pronounced with the /ɨ/ (i), and it never occurred to me that this pronounciation (which I have adopted) might be considered rural (and I really, really believe that's just a normal regional variation, I doubt many people will notice the difference in fast speech anyway). In fact (it's really hard to find samples of such words on m-w), even though the final syllables of "undaunted" and "imbedded" are both described using /ə/ by m-w, their samples of both words clearly sound like having /ɨ/ to me. Do they sound strange to you?http://en.wikipedia.org/wiki/Unstressed_vowelhttp://en.wikipedia.org/wiki/Close_central_un...
Rant: Both dictionaries you quoted are utterly wrong in using square brackets to enclose their pronunciations. Dictionaries shouldn't give phonetic descriptions (which describe the actual pronounciation that can be very different depending on the accent) but phonemic descriptions (which is the underlying description and relatively constant across dialects). Dictionaries that don't even get this simple distinction right shouldn't be trusted at all. And god knows why BrE dictionaries often use /e/ to describe the phonemeic representation of the vowel in 'bet', which is so much closer to [ɛ]/[ə] than to [e].
@Jenny: Careful, "envelope" may be pronounced /ˈɛnvəloʊp/ or /ˈɑnvəloʊp/.
PS. I know, I should try to avoid parentheses.