Would the HD598 sound better with the o2+odac?

I just ordered the HD 598 and I have an ok sound card but I was wondering if the o2+odac from would sound better than my current solution.

They would sound somewhat better with a amp+dac, though it probably won't be enough of an improvement to justify the cost unless you are planning to get further into high end audio in the future. If you do decide to get a dac and amp, I strongly recommend that you go with a Schiit Magni + Modi over an O2 + Odac unless you are planning to build your own amp. The Schiit combo will give you marginally better sound quality for about $50 less.