Is taking valium and oxycodone at the same time generally considered safe?

Not really: Both valium and oxycodone have respiratory depressive effects which can be compounded taking them together.

This can create: Profound somnolence, sedation and dysphoria...As well as GI upset, constipation...And not advised unless prescribed specifically by medical personnel and for severe pain postoperatively, etc.

It is a risky : Choice. The likelihood of overdosing and having a respiratory arrest will increase when you mix these medications. Only do so under the direct supervision of your doctor.