Something has gone badly wrong with football: a few years ago there was a lot of hand-wringing about what to do about "professional" fouls, where a defender cynically brought down an attacker who was clean through on goal, but outside the penalty area. It meant that a situation which might be a very likely goal was transformed into a free kick, with defenders allowed back behind the ball, and the culprit was roundly despised in the press and by the opposition, but feted in his own dressing room as a diamond geezer :-)
To counter this, a rule has been brought in which allows - demands - a referee to send a defender off if he's deemed to have committed a foul as the "last defender". I am assuming the OP refers to Arsenal v Chelsea game, where Costa was fouled and by the rules of the game, the defender had to be sent off. The "Oscar" comment presumably refers to the theatrical rolling around on the ground, just to make sure the ref got the message. But that sadly is par for the course in football these days.
However, the foul by the Arsenal player, like most of those similarly committed and punished, was hardly a "professional" one. it was simply clumsy.
The sensible way to address these situations always seemed to me to be to award a penalty, tho the foul was outside the box. Ideally only in situations of blatant cynical foul play. What we actually have are situations where a team can be reduced to 10 men for the remainder of a game just for committing a technical foul. Moreover, if the offence occurs in the box, the sending off STILL occurs as well, even though a penalty is also awarded. This is particularly ridiculous in the box, where I've seen situations where a defender can't possibly have known that he was marginally the "last man".
So as far as I can see we've ended up with a rather silly rule.