*The anime definitely focuses more on establishing Andre's feelings for Oscar out the gate, and a lot of scenes in the manga that happen without his input have him giving monologues in the anime. He and Oscar saving the little child in Aras and bonding while there, as well as him having the nightmare that Oscar doesn't need him anymore the night before Marie's horse runs wild are both exclusive to the anime, for example.
*There's plenty of other scenes that were changed and lengthened in the anime. Like the duel with that Duke that she shoots in the hand in the anime (and straight up kills? in the live action version) in the manga it looks like Marie gives her the suspension just for arguing with the duke before a duel can happen. The anime's tone is definitely darker and more serious overall, as the manga often incorporates cartoony caricature imagery for humor, and lofty "greek goddess" imagery (not sure how else to describe it) for romantic moments.
*Rosalie's adoptive mother has black hair in the manga, making her look more like Jeanne, which makes sense. There's also a scene in the manga where Jeanne comes back to pack her things after the rich woman agrees to take her in, and she tries to convince Rosalie that exploiting her noble blood is the smart thing to do.
*Fersen's sister Sophia is a minor character in the manga. She was not in the anime.
*We don't see so much of Charlotte - and even less of her fiancé - but she still commits suicide. In the manga, Oscar is right there and tries to grab her before she falls but fails.
*Andre puts up a fight in the manga when his grandmother wants to cut his hair so he can impersonate the black knight - as opposed to the anime, where he does it himself after seeing Oscar in the costume.
*We see more of Rosalie in the manga - and so her romance with Bernard makes more sense. In the manga, Bernard as the black knight kidnaps Rosalie, and that's why Oscar goes to Palis Royale, to try and save her, not because she's trying avenge Andre's eye. Andre ends up saving both of them and... Rosalie ends up shooting Bernard as they escape. Back at the mansion, she is the one to tend to him. She reminds him of his mother and they fall in love.
*The anime does a better job of establishing Girodelle as a character so that we actually know who he is when he proposes to Oscar - in the manga, he has like one tiny cameo before that. Actually, in the manga the party Oscar goes to is her engagement party to him specifically (not just a party for her to find a suitor). Oscar goes in her military clothes and flirts with all the women (even kissing a few!) just to show how much she doesn't want to marry Girodelle. Shortly after, Andre, who is loosing his sight and distraught at his and Oscar's differing social classes after a conversation with Alain (who is also in love with her in this version!) nearly commits a murder suicide by poising his and Oscar's wine, but ends up telling Oscar not to drink it.
*The manga does a better job of establishing that Marie's kids loved playing with Oscar, and this is why her son had a crush on her before his death.
*Oscar's confession of love to Andre happens at least a few days before their famous "love scene", not immediately before, like in the anime. In the manga she confesses to him right after the scene where her father is threatening to kill her and Andre intervenes. Then they hug/kiss and there are some scenes of them hugging and kissing more building up to their "wedding" night - which in the manga happens in her room, not outside like in the anime...
*There's a cute scene before all that where Oscar sees Andre with his shirt off and doesn't understand why she's so flustered.
*After Andre's death, he seems to come in (angel?) form to "pick up" Oscar, and they both then go to "pick up" Andre's grandmother, who was sick and bedridden for awhile at that point, when she dies shortly after they do.
*In the manga, instead of Andre pretending to be able to see the painting of Oscar and describing something different, he simply thinks "I want to see it!" and then Oscar starts coughing up blood, tipping everyone off that she's sick.