Brilliant move help

https://www.chess.com/analysis/game/live/63758439325?tab=analysis

In this game move 5, O-O is brilliant. Can someone explain why? It doesn't make any sense to me.

The game report (thing with accuracy and Brilliant and other types of moves) was been back and forth with experimenting ever since it was created. It is often misunderstood, but this feature was never intended to be a deep analysis tool for high level statistics; it was supposed to be a quick "hit-or-miss" tool to help identify key moves for deeper analysis on your own. The "Brilliant" moves and such as to help motivate newer players into asking questions and learning and the "Blunders" (and "Mistakes" etc.) are supposed to be just guidelines to get the player identifying moves they might improve upon. 

Thank you all so much! Very helpful. I'm only 500 rated so this type of stuff helps me a lot. Appreciate it guys!

I will do that and see where it gets me. Appreciate the advice. I've been stuck down here for like 6 months lol

In that case, this and little else is all you need for the opening stage of the game:

https://www.chess.com/blog/KeSetoKaiba/opening-principles-again 

Feel free to message me if you have any questions, need help, or just want to chat.

That also goes for non-opening chess things or even other topics too. See ya around

https://www.chess.com/analysis/game/live/63905512393?tab=analysis

1. d4 e5 Standard trap I play against 1.d4. Looking for 1. d4 e5 2. dxe5 Nc6 3. Nf3 Qe7 4. Bf4 Qb4 or 1. d4 e5 2. dxe5 Nc6 3. Nf3 Qe7 4. Bf4 Qb4+ 5. c3 Qxf4 which was what happened in this game. 2. dxe5 Nc6 3. Nf3 Qe7 Bf4 Qb4+ 5. c3 Qxf4, which was what happened in this game. 2. dxe5 Nc6 3. Nf3 Qe7 4. Bf4 Qb4+ 5. c3 Free biship 5... xf4 6. Nd4 Nxd4 Trying to simplify the position by capturing knight. Had thought after xd4 cxd4 and xd4 free pawn, but forgot the queen defended. Silly mistake. 7. cxd4 Qxd4 8 Qxd4 1-0

A lot of the time while I'm playing I just don't observe the whole board. I think I'm pretty good at seeing 2-3 moves in, for example if I'm analyzing a potential move, but then I just end up blundering something stupid, for example in the game you just showed. I've been analyizing my games and I've noticed this pattern a lot, for example in this game.

PS. How do I show the games in the web browser lol.