LOL. A really good OP will give us an idea of what the show is going to be like, and provide clues that we can look back on later and think "wow! I can't believe I didn't notice that!"
Some good OPs take it a little farther.....and literally ruin the whole show if you're paying enough attention :D
A few examples...(and SPOILERS. Obviously. Given on the title of this card XD)
Yamada-kun and the 7 Witches
I wonder who the 7th one is? 🙃🙃🙃 It wasn't exactly hard to know who were the Witches anyways, but the OP kind of gives it away.
adfka;d!?? Why!!! Lol. XD I'm not going to lay out what this spoils for those who haven't seen it (maybe they'll just think it's cool animation, haha)
Love Live S1
Does this count as a spoiler? Idk, if you ever thought they'd really disband then yes XD It basically shows the ending! It also lets you know who's gonna be in the group, haha.
Fairy Tail OP16
Actually basically any Fairy Tail OP realllllly tells you what's going to be happening XD Not that I dislike, but it tells you a lot before it even happens!
Mirai Nikki OP 1 & 2
This is only a spoiler if you REALLY thought into the OP, but the OP pretty much told you a big thing that happens later on if you're paying attention!
I don't really mind that OPs "spoil" things!
Sure, I wish they were all more subtle, but I love when you can come back and watch an OP like ERASED's and appreciate it so much more because you know everything that you didn't know before. It's not spoiling as much as it's foreshadowing, and when the same OP is used throughout the series, you're able to get more and more from it after each episode, which is pretty cool in my opinion!