We have seen Kikumaru playing doubles with other people, and winning. But we have only seen Oishi with Kikumaru. That is the key about downgrading Oish. He has been historically better than Kiku (he was called to play with the regulars while he was in 2nd) and is style hasn't changed. The problem is that Oishi's abilities aren't really spectacular like Kikumaru's clones.
He is the master of doubles, because he only plays doubles. And he dominates all the formations and that stuff. A formation isn't something as nice to be seen as a serve or a volley, but it's more effective.
We have only seen one doubles match of Oishi without Niou. And the executor was Nioh, but the mastermind and the one who controlled the full freaking game was Oishi again.
To be honest, Oishi didnt 'control' the game.
He controlled the win however. I accept Oishi earned his win.
And also, it was stated that the only 3rd Year who wasn't a regular in their 2nd Year was Kawamura.
By 2nd Year, Kikumaru and Inui were also regulars.