We have reched gold campaign level in sob (460 CP). Our OL has 2 cities and 2 more are going to be razed. He is playing the queen plot (no alchemist, double cost for weapons, etc.). He has played a card to destroy automatically a city, so we are going to lose hopelessly. This is frustrating because we were prepared to defend the fifth city from lieutenants attacks, and we were cautious with his movements.

I think that we should had known wich cards could he play along the campaign to act with enough time. This is a high strategic game, and I think that it is very difficult to win if we are not prepared against his plots and/or avatar upgrades. He did not let us to see plot cards.

Do you think that plot and avatar upgrade cards should be seeable by the heroes to act with concordance? Or are the cards observable only for the OL?

The only rules about hidden cards I'm aware of at all are, the OL's card hand, and the Feat cards for heroes (which some people translate to mean keeping hidden from all other players even other heroes, I'm not sure if this is a rule but I don't play it that way).As for decks, only ones like the OL's draw deck during play, treasure card decks (after a few games most heroes will know what are in them), any dungeon or rumor decks, encounter decks etc... but I don't think anything says the OL's plots are hidden, anymore than his Avatar sheet or his bought upgrades, or Ltn's. Even Upgrade cards are double sided repeating all the same text so they become obvious no matter how they are placed. I don't think that you aren't allowed to look at the future plot cards, at least not as a rule.

The only text I can find refering to it...

"Once the overlord player has chosen a Plot, he reads the flavor text on the zero-cost card associated with that Plot, and puts it into play (i.e., faceup on the table)."

Likely the rest are set aside in a stack with the rest of the OL's Avatar upgrade cards and Ltn's, none of which are not allowed to be viewed by the players. Thematically it might not play into things well, but since Descent is a game where anyone can play the OL, unless a group uses specific house rules, or there will only ever be one OL, then what's the point? Even after, if you play it once, any other time you play you'll be more aware of it.

+1 for Veritech's answer.

I can understand why your OL player would want to keep the plot cards secret (not even for the effects but just for thematic reasons,) however, there's nothing in the rules that says those cards are kept hidden. The only rules I'm familiar with are the ones that say each card is "read aloud" once purchased.

Considering how notoroiusly poorly balanced SoB is, I'd say your request is even more reasonable here than in RtL. (It would still be reasonable in RtL.)

the OL cannot hide his plot cards from you, even if unpurchased.

In a game where none of the players has any previous experience in advanced campaigns I would think it would be good to discuss this in the group, whether or not players would like to know in advance. In this case I do not think the advantage would be too grreat, as people are very much learning. But after a game or two I would suggest both hero and overlord should have at least a basic overview of the available possibilities.