Post was planning to release some on-the-go versions of their cereals under a brand called "OK GO!" This seems like a pretty reasonable thing to do. I certainly don't feel like they chose that name for the purpose of somehow confusing consumers into thinking their cereal was related to the band (nor can I imagine anyone accidentally making that mistake).
OK Go apparently threatened to sue Post ("Post said OK Go had been quietly threatening to sue for months, claiming that the company had infringed the trademark rights to the band’s name by launching the new on-the-go packages earlier this month.), but didn't actually do so. Post filed a lawsuit that didn't request any kind of damages, just preemptively asked the court to rule on whether their new cereal brand infringed on the band's trademark ("Post is seeking what’s known as a “declaratory judgment,” meaning a ruling by a judge that says the company did nothing wrong.).
That seems like a perfectly reasonable thing to do - there's a dispute over whether Post can use the name, and Post is just trying to get clarity on whether it's okay or not from the court.