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Wikipedia lists some rules and suggests that the plural form of olio is olii.
I used to think that oli sounds (and writes) better, and Dizionario d'ortografia e di pronunzia confirms that.
Treccani, however, suggests yet another plural form, with circumflex: olî.
Any rule of thumb I could use?
We can derive a general rule (with loads of exceptions, like everything in this language) from this case. When the expected plural form of a word contains a non-stressed double i, like "olio" -> "olii", "principio" -> "principii", there's a tendency to drop the second i: "oli", "principi" (Ngrams for "olii" vs "oli" confirm this graphically). However, some speakers do retain a memory of the double i in their pronunciation of such words, the i is slightly longer.
To tell a "single-i-that-used-to-be-double" from an ordinary single i the circumflex accent can be used and this is standard Italian grammar. This makes sense particularly when one needs to disambiguate, for example "principî" (plural of "principio", principle) as opposed to "principi" (plural of "principe", prince); this is the solution used in the Italian Constitution that includes a chapter called Principî fondamentali. Another solution in this case is the tonic accent: "principî" can be unambiguously written as "princìpi" because the other word is read as "prìncipi".
Conclusion: the three forms ("olii", "oli" and "olî") are equally acceptable, but as you can imagine the circumflex accent is unusual and most people wouldn't even be able to type it on their keyboard. AFAIK, this is the only case in which a circumflex accent can be used in Italian words not borrowed from foreign languages or dialects, I guess most Italians wouldn't even realise that in their language this accent exists.
Notice that the situation is entirely different with stressed plurals in -ii (e.g. "sgocciolìo" -> "sgocciolii"). In this case the double i is pronounced clearly by all speakers (the first i is stressed) and is compulsory. The opposite applies to nouns ending in -cio, -glio, -scio etc. where the i is not really used as a vowel: the plural form requires one i.
Using î instead of ii is something that was once done, but nowadays that is not done anymore, at least on everyday usage. Between olii and oli, I would expect the latter to be used more often than the former, at least because it requires you to write less letters, and generally there is no confusion about what oli means. (It would not be taken as plural of olo, since that is not an Italian word.)
Lo Zingarelli reports just two plural word for olio: oli and olii.
i either...
I'd add that if an adjective follows, like "oli vegetali", the form with a single "i" is commoner (curiosly, a quick googling shows that "olii vegetali" is mainly used by people which write about organic products and the like)
One should always check and double-check what Wikipedia says, but in this particular case, I'd agree with its distinction between compulsory -ii in the case of a stressed i in -io, and multiple possibilities (-i, -ii, and -î) for an unstressed one.
olj, which Pirandello would probably have used.
In Italy we never use olî. Also olii is not so common, even if it's better than oli.
