on is a singular for we. It takes an auxillary verb in the singular; therefor the past participle should be single also so it should be on est allé

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Hi,
I agree with Peter that it is a strange one but as odd as it may be, 'on doit l'accepter'.
Just to recap -  
1.  When  'on' means 'we', the past participle will agree in gender and number to whom it refers to:
Marianne et moi (Cécile), on est allées au Portugal pour nos vacances d'été.
On est sortis en bande avec des copains hier soir.
On est rentrés tard hier soir après une bonne journée passée en famille.
On s'est séparées à regret, mes amies d'école et moi.
2. When 'On' stands for 'tout le monde', 'les gens', 'en général' , it will stay in the third person singular as in:
On n'est jamais si bien servi que par soi-même = If you want something done right, do it yourself
Quand on a reçu un don, on a des obligations =  When you receive a gift, it creates obligations
Hope this helps!

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Hi Peter,
there are numerous posts and explanations under this topic already. Can you point to what explicitly you don't understand?
-- Chris.

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I do not understand why the past participle is plural when the modifying verb and indeed the subject are single. In general French is a logical language and single subjects take single verbs not verbs which are part single and part plural and I was never taught this exception

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Well, what can I say. A language is not math. 
-- Chris.

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thanks this is a very lucid explanation  merci

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Like you Peter, I was a bit taken aback to learn that ‘on’ could be used in a plural way. I distinguish between them by thinking of the singular ‘on’ in terms of using the reference ‘one’ does this or that. At one time it was quite commonly used (on pun intended, lol )

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AUrélie,  Laura says below that the use of “on” depends on whether one speaks of specofic people or “groups, mankind et al in GENERAL”.  athat I u derstand.  But when the phrase revers to (for example) Melanie et moi, the reference is to 2 specific people.  Why do say “on est allées..” instead of “nous sommes allées”?  What is the very subtle difference in meaning and usage?  In English the use of “one” did or said etc has decreased and is now looked on as being a somewhat pretentious way of speaking.

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Bonjour Marnie,The only difference is this: nous sommes allées is normal register and on est allées is informal. So in essence, you'd always use nous when talking to people you vouvoie and probably switch to on with people you tutoie.

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Merci Laura.Sometimes one needs to know the reason "WHY" an expression is used in addition to knowing HOW, WHEN and WHERE it is used.  It's the difference between memorizing a language and understanding it.  It would be interesting to know how this usage evolved...As non-native speakers we need all the help we can get!

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I’ve been wondering about this topic myself and after poking around a bit, here are my two cents: strictly grammatically speaking (go ask the Académie Française if you don’t trust their website) you are never wrong if you leave on as masculine singular. That is why, historically, there was no agreement with on. Fin d’histoire.

That said, not everyone cares what the Immortels say about the language, so people like us, whether writers, native speakers or francophiles, make our own adjustments to the language as time goes by. If they make sense to enough people, they tend to stick- at least for a while- and may fall back out of favor down the road. Language only lives on by evolving with the people who speak it.

A professor of mine in Paris used to talk about various exceptions in French as being a matter of poetry because something did not suit “le délicat oreille français”. I think the case of the agreement with on is similar, although based on an affront to logic. On some level it doesn’t make sense to use on to designate a clearly plural subject and not make a necessary agreement. From the posts above and what I found on the web, there is a solid logic to the modern agreement with on and I’m sure it will live on in the French language until someone can argue soundly to the contrary.

Vive le débat!