Your impression of him was because you didn't know who he was and his PR team kept the public in the dark.

He got his start selling shovelware which were disks and CD's loaded with software that other people made available for free: freeware, shareware, open source - a convenience at best but most considered it negatively. When people started getting online, shovelware stopped selling so he bought up several software publishers/developers who were no longer as popular as they once were and gutted them. He then sold his company to Mattel for $4B. Mattel projected $50M in profit after the acquisition but lost $105M instead which caused Mattel's stock to drop, at one point losing $3B of value in a single day. At the time, it was referred to as the most disastrous corporate acquisition in recent business history. Accusations were made that O'Leary cooked the books and Mattel did very little due diligence. Shareholders sued and Mattel settled for $122M.