Last I checked, Tianhuo forward dash could go under Oleander's air fireball and ground fireball. Because it's a normal dash and not a special movement option, Tian can also block while advancing, so if Oleander drops a trap on the floor or stops shooting to poke her with the book, Tian is still generally safe.
From fullscreen, Tian can just stay on the ground, walk forward, and dash under fireballs on reaction. Oleander can't do anything to a grounded Tian from far away. And it's almost impossible for Oleander to gain distance with her forward teleports, so you'll push her to the corner eventually, and then she has no easy way out.
In theory, Tian vs. Oleander should be a hard fight for Oleander if Tian is patient and stays on the ground at long range. You should make your openings by baiting Oleander into whiffing a poke or chicken-blocking something (aerial block very close to the ground; landing will cancel your blockstun and you can act as soon as you touch the ground), instead of just charging at her and trying to start block pressure.
That's all in theory. I need some actual experience with Tian vs. Oleander to say anything about how this goes in practice.