The reason why Orianna is "OP"
Orianna was always considered a balanced, slightly strong pick up until recently. Why have people only recently? The meta has shifted in her favour, games are longer, previously supposedly "strong" champions like Ahri have been nerfed slightly and people are realising that Orianna has actually always been very strong.
Good Late Game and Shove
In addition, she can also shove the wave well to also avoid trading with enemies she beats late game but loses to early game. Her shoving power and good CC + Utility also gives her decent roams.
She can, at the very least go even against any matchup, going even isn't a punishment for Orianna either because of her good late game ult.
Safer more reliable Poke than many mids
Orianna's poke is much more reliable than many other mids. Ahri's trading requires enemies to misstep into her charm, or misstep into her Q which shouldn't hit otherwise. Anivia requires enemies to have extremely slow reaction speeds (until she reaches 6 where she becomes a god). Leblanc needs enemies to not be too close too minions, if they are too close, she can't chain them or W chain them without getting into a dangerously close range. She could W them if they are close to the minions, in order to hit both the wave and the minions, but she will never proc the sigil because the enemy will have walked away from the wave so the Q doesn't bounce to them.
Orianna on the other hand, will either permazone you from the wave after she moves her ball to the casters, or you farm, and as soon as you stop to auto attack, she Q Ws you and runs away. You will not be able to punish much her because of her decent range, shield and no cast times. She will already be in a reasonably safe range and be able to run even further away because of her lack of cast times. Most of the time, without mobility, you will not be able to avoid the Q W because of how close it is.
Safe Farming and Excellent Freeze
If you do have an extremely good farming range and won't be hit by Q Ws as easily, i.e. you are Vel'koz or Taliyah, then she can just try to shove or at least match the shove and wait for late game where she will often shine brighter because of her ult and utility. If you have extremely good shove that Orianna can't match i.e. you are Anivia, Orianna can use her shield to tank the minions to create an oppressive freeze without losing much health as punishment.
Overall
Orianna is "OP" because of how safe she is. She has a potentially better late game than many of mids and she has good waveclear. If she is against a matchup in which she has a better late game, because of her effective farming range because of her low cd Q and her good wave clear, in addition to her lack of cast times, she can farm extremely safely from a distance because of her Q and avoid lots of punishment and trading. Just because she was not considered "OP" doesn't mean she never was. Champions are "OP" in different ways. Orianna's way of being "OP" is lots of utility in her kit, good shove, reliable poke, good farming, good late game, some mobility from lack of cast times and move speed boost. She can beat almost any mid in some way because of how good she is at everything and how safely she does everything.
