> Ianal, but afaik in general your employer owns the ip, and as such publishing it as oss requires explicit permission

If any of the work is related to what you do for your job this is true.

If the work is not related to the job it depends on the state. Many states have limitations on what employers can claim as their IP. Generic contracts will try to claim everything because they keep the language broad, but laws often say that an employer can't claim work you did in your free time if it wasn't related to the employer.

If you do the work during work hours or you use the company laptop, they would have a claim to it. Most companies aren't going to care, but you shouldn't get relaxed about this because you want to keep everything clean if a dispute arises.

Do the work on your own time, on your own hardware, and don't overlap the work you're hired to do or anything you might have been exposed to during your time at work.

Yes, but the title of this page is literally "Keep OSS alive on company time".