An adjective phrase and an object complement are two different levels of description, so it is possible (indeed common) for an object complement also to be an adjective phrase.
Here's a simpler example to explain what I mean:
The big man read a long book.
The big man is a noun phrase (one level of description, explaining the grammatical category of the words). It is also a subject (a different level of description, explaining the syntactical role of the phrase).
Now we need to look briefly at what an "object complement" is. We'll start with a "subject complement", because it's easier to explain. A subject complement is some kind of phrase (which may be a single word) which gives information about the subject of a particular type of verb, which we might call "existential" verbs. These are verbs like "to be", "to seem", "to look", which express something about the state of the subject:
The man is tall.
The book is long.
In these cases, tall and long are subject complements: they tell us something about the subject, rather than being objects of the verb. They are not affected by the action of the verb, but they expand on what we know about the subject. Contrast "the man buys a book", where "a book" is the object of the verb "to buy": the book is affected by the action of the verb, but gives no information about the state of the man.
An object complement is like a subject complement, but it gives extra information about the state of the object of a verb, not its subject. So instead of following one of these "existential" or state verbs, it follows the object directly, with the sense of "to be" or a similar verb implied:
They made me angry.
The structure here is subject (They), verb (made), object (me), object complement (angry). Structures which include an object complement like this often (though not always) contain verbs which express doing something physical or abstract to the object:
They called me an idiot.
They painted it red.
So, "object complement" is a structural category, explaining what a word or phrase does in a sentence.
In contrast, "adjective phrase" is more of a formal category. It explains the grammatical category of the key word in the phrase.
So in the example you give, you could argue that we have an adjective phrase ("off guard") which is performing the role of object complement in the sentence - the object being "me".
There are various arguments for and against this analysis, but the answer to the original question is, in summary, that it doesn't have to be one or the other; it could be both.