I also want to ask you how they treat you normally.
If you've never noticed anything particularly unfriendly, then it is very likely an oversight. And yes, oversights shouldn't happen, but in reality they can and do all the time, and if is was an oversight, it's really not worth stewing over and building into a big deal in your head.
Do you feel you need answers about it all or can just live with it? If you need to know what happened, you will have to raise it with someone at work. It doesn't have to be a big dramatic and hurt kind of conversation - a very light hearted comment that you saw the photos if the party and it looked fun will fine.
From this the peson might ask why you weren't there, and again I think it's important to not make a big thing of it or to be hurt, but you could simply say you didn't know anything about it.
If you feel you can ask more directly, start with the comment about it all looking lots of fun and follow up with the fact you did t know anything about it - this sounds less critical than 'I wasn't invited' - and then there will be an exp,a nation of why you didn't know;
- it was organised before you started and no one thought to check the list
- it was just for certain staff
- there was a mistake made and you were missed off
- it was an Event for a particular group of workers, not everyone.
You can also say that it looked fun and you'd be up for that kind of thing next year.
Definitely best to keep it all light hearted, because what can be achieved by seeming upset and hurt by it all. By having a conversation you can make clear you are aware it happened and you didn't know about it. In all liklihood, the person you speak to will say there has been a mistake or if they don't know what happened, mention it in passing to the organiser who will then realise a mistake was made and hopefully apologise to you.
If they tell you there was a mistake or if someone apologises, I think it's important to be gracious even if you felt hurt. These things do happen and it's not normally intended as a personal slight and it's really important not let take offence when it's not needed.
If of course you were deliberately excluded (unlikely) then that is different. If this were the case, I think you would have already got the vibe of being unpopular already in the office....and as you haven't, I think you can probably relax and not worry about being disliked. The worst that has happened is probably an oversight....yes, a bit thoughtless, but really important not to let a bit of thoughtlessness turn into a bigger deal in your mind and eat away at you.