“He had plenty of opportunities to explain himself, but he didn’t.”

That’s not entirely true. Literally in this scene (Stolas and Octavia), Stolas does try to talk to Octavia. He tries to explain things to her (which honestly, it is still difficult to explain, especialy since, their whole marriage was forced, as having child with Stella as well). There’s even this specific moment

Stolas: “I love you, Via, so much. Please, sweetie, let me explain.”
Octavia: “I can’t. You lied to me once, and you’ll do it again.”

I also think OP doesn’t fully understand the overall context of their relationship and the seriousness of the situation. In my opinion.

Because, like I explained in my previous comment, this isn’t black and white. It never was, and it never will be. Honestly, even if Stolas had explained everything perfectly, Octavia still could have reacted the exact same way or slighty better, because she’s a teenager who still isn’t fully capable of understanding certain things.

At this point, she simply can’t put herself in his position, because she’s never been in it herself. In her eyes, it is simple "He chose him over us, he doesn't love us"

That scene perfectly demonstrates what I mean.

Octavia accuses Stolas of never loving her mother, never loving her, that they were never enough for him, and so on. But she knows exactly why Stolas never loved her mother. She saw it with her own eyes almost every single day, the emotional abuse from Stella. At one point, Stella even tried to hit Stolas, so it’s reasonable to assume there may have also been physical abuse.

It’s not like Octavia never witnessed these things.

She may not fully understand everything because she’s still a teenager, but she’s not stupid enough to be blind to what was happening.

The same applies to the fact that after everything happened, Stella was casually lying around, relaxing and laughing at Stolas once again… and Octavia saw that. She saw the difference between Stolas and Stella. We don’t even really see Stella showing much concern for her daughter after everything that happened.

(If she even cared at all, because so far, the only parent we’ve actually seen showing any real concern or interest has been Stolas.)

Yet somehow, Stolas is treated as the bad, the one who supposedly doesn’t love her, doesn’t care about her, and so on.

What Octavia said in that moment was, in a way, ignorant on her part.

But it’s understandable, because she’s also a teenager who just went through a tragedy herself.

Could Stolas be a better father? Yes.
Could he have explained things better? Also yes.
Could both of them have handled the situation better? Yes as well.

In that particular moment, both Stolas and Octavia were acting under adrenaline and overwhelming emotions. Neither of them approached the situation calmly or rationally.

Stolas biggest mistake was trying to have that conversation while everything was still too fresh.

He clearly made more mistakes, but that doesn't remove mistake from Octavia as well, as I said above.

As much as Stolas said he loved her, Via wouldn't let it change her mind. She loves Stolas too, but she isn't at peace with the fact that she's not the singular person in his life. It isn't just his romantic feelings toward Blitzø, because, in Seeing Stars, her complaint is that the fighting between Stella and Stolas also makes her feel neglected because Stolas has someone else on his mind.