In the anime and manga we cannot answer this question, as the two have never actually faced off. However, a confrontation has occurred in the webcomic*.
You can "watch" the entire (translated) confrontation on YouTube, in fact:
Part 1
Part 2
The basic rundown is as follows:
Not long after the Garou situation, Tatsumaki aggressively confronts Fubuki when Fubuki tries to obtain further information from the Monster Association's leader (Tatsumaki simply wants her dead). Saitama intervenes when it gets a bit serious, grabbing Tatsumaki's hand to stop her. Tatsumaki aggressively tries to blast him away. It fails and he doesn't let go. She tries to bend his very energy to take him out. It fails to do much of anything. She tries to knock him loose, dragging him halfway across (and along) the countryside with telekinetic flight, but fails. At one point she opens a large chasm in the earth and lets Saitama fall into it, then reseals it and buries him there. He effortlessly digs his way up like a land shark. At another point, she tries to lift Saitama up telekinetically, to hurl him into space, only to find it takes extreme effort to lift him even a foot off the ground. Saitama enjoys the experience of "flying". Ultimately Tatsumaki concedes that Saitama is extremely strong, and is aggravated with how he "hides" his true power. However, she insists that if she were at full power—she has already resumed bleeding heavily from her head from the injuries she suffered during the Garou arc, and is possibly concussed—then she would have "easily" crushed Saitama. Saitama seems...unconvinced.
*Since some people seem unaware of this, One Punch man originated as a webcomic (chapter links are at the bottom). It is the fundamental canon of the story, from which the manga and anime are derived.