TV Series:
After three subsequent meteor crashes on Earth the Earth Federation Government is formed. Researcher Bian Zoldark found out that the third meteor is not of natural origin and contains alien technology within, the so-called Extra-Over-Technology (EOT). Seven years later EOT led to the development of military-style Mecha, called Personal Trooper (PT). But now unidentified mecha are mysteriously appearing all around Earth...
OVA:
Set after the events of SRW Original Generation 2 on the Nintendo Gameboy Advance, an unknown fleet of enemy mechs called Valtors suddenly attacks the populace of Earth, destroying all opposition, with millions of citizens instantly vanishing during the raids. ATX team members Kyosuke, Excellen, and Bullet, along with Ryusei, Rai, and Aya of the SRX team are sent to investigate, before the situation becomes more dangerous than ever.
I myself have seen the OVA and a bit of the TV series, but I'm mostly familiar of the SRW OG world because of the GBA games.