In that Indy ended up trading Oladipo, I think it is fair to deduce that he could have been available to the Celtics. Pre-Opt out, I forget exactly what Hayward had for a contract but it would have had to have been Oladipo and something for Hayward. But even this, you can't say for sure. They traded him for LeVert, a really nice young player on a competitive contract.
Anyway, if you want to assume that we could have gotten 1 year of Oladipo (before he signed with Miami), I guess that is your theoretical missed opportunity. But that would have been it likely. He would have put us at a higher cap level and we could not have signed much else. Instead we got Teague and Thompson and still potentially some additional help through TPEs.
I would have been OK if we ended up with 1 year of Oladipo but I don't see it as some major missed opportunity. And more likely (at least in my mind), Turner and McDermott is the missed opportunity. No one knows if Oladipo was actually offered.