Oscar alternates between using his sword two-handed and one-handed and kicks for heavier stamina damage. His attacks deal decent damage, but he also moves slowly for an NPC enemy, allowing for plenty of opportunities to heal. He has the heavy roll, making it easy to strafe him.
Oscar was likely a high-ranking knight from Astora, who was determined to seek out Lordran once he turned Undead. Exactly how he ended up in the Undead Asylum and why he rescues the player is unknown; no other characters make mention of him.
...Oh, you... You're no Hollow, eh? Thank goodness... I'm done for, I'm afraid... I'll die soon, then lose my sanity... I wish to ask something of you... You and I, we're both Undead... Hear me out, will you?
...Regrettably, I have failed in my mission... But perhaps you can keep the torch lit... There is an old saying in my family... Thou who art Undead, art chosen... In thine exodus from the Undead Asylum, maketh pilgrimage to the land of Ancient Lords... When thou ringeth the Bell of Awakening, the fate of the Undead thou shalt know... Well, now you know... And I can die with hope in my heart... Oh, one more thing... Here, take this... An Estus Flask, an Undead favourite... Oh, and this...
...Regrettably, I have failed in my mission... But perhaps you can keep the torch lit... There is an old saying in my family... Thou who art Undead, art chosen... In thine exodus from the Undead Asylum, maketh pilgrimage to the land of Ancient Lords... When thou ringeth the Bell of Awakening, the fate of the Undead thou shalt know... Well, now you know... And I can die with hope in my heart... Oh, one more thing... Here, take this.
...Yes, I see... Perhaps I was too hopeful... Hah hah... Please, leave me be... I have not long to live, and I may harm you after death... Now, go...
...Now I must bid farewell... I would hate to harm you after death... So, go now... And thank you...
In the credits, he is acknowledged as "Knight of the Undead Asylum". His name is not mentioned in-game; however, he is identified as "Oscar, Knight of Astora" in a text dump extracted from unused game files.
In the room the player finds Oscar in, a hole can be identified in the ceiling. Before the player triggers the boulder no other possible way in or out of the room is evident, making it likely that Oscar was fighting the Asylum Demon, since he appeared well and able not long ago and is suddenly nearing death when the Chosen Undead discovers him. This is hinted at further upon opening the first set of doors to the Asylum Demon's arena, where it can be seen waiting for the player on the destroyed roofing.
Oscar can assist in battling the Black Knights, thus making him also vulnerable to their attacks.
The following is content that is present within the code of Dark Souls, but not actually implemented in the game. Text was found from a text dump which can be found at: http://pastebin.com/hA42TwK8.
Click to see Removed Content (Contains Spoilers)
Oscar was originally meant to play a much larger role in Dark Souls. His removed dialogue suggests that he was to be a summonable in Anor Londo and the Darkroot Garden. In addition, he seemed fated to confront the player at some point in the game.
Speaking to him in an unknown location after ringing the Bells of Awakening
A pleasure to meet you. Sage Frampt has spoken to me about you. I am Oscar of Astora. I wish to thank you for ringing the Bell of Awakening. I have received the word of Sage Frampt. I will be fine. You ought to focus on yourself. May we each fulfil our respective purposes!
Speaking to him again in the unknown location
I am preparing to leave. I will follow Sage Frampt's instructions, and will seek Anor Londo by way of Sen's Fortress. I hope that we meet again somewhere, one day.
Speaking to him in Darkroot
You must be the same as I; in search of the grave of Sir Artorias. But, be careful. This forest is the territory of a fierce band of thieves. They assault any and all who seek the graves...What if we were to join forces? How about that? If we worked together, we may escape the bandits. Does that appeal to you?
Very well. Let us keep our wits about us. One can always do with another pair of eyes...
Yes, I see... No matter. May we meet again. I pray for your success.
Speaking with him after cooperating in Darkroot
Things appear to have settled. I was right to partner with you. Thank you. We don't need to band together any more. I will have a short rest here. Feel free to go on ahead...
Speaking with him after placing the Lordvessel with Frampt
I have waited for thee... Foolish slave of the Gods, and pawn of Frampt... I will kill you... And become the true Dark Lord...
Speaking with him after placing the Lordvessel with Kaathe
So, it was you... I had a feeling... I shall destroy you, as fate has commanded me... Foolish pawn of Darkstalker Kaathe... And fiendish Dark Lord...