Can I renew my expired Belgian passport if I have since taken US citizenship oath?

First of all, whether you have Belgian citizenship or nationality and whether you can get a Belgian passport are all solely matters of Belgian law. The US has no say over it. Nothing you do in the US or with respect to the US can have any effect on your Belgian citizenship except as provided by Belgian law. @chx's answer covers whether acquiring US citizenship causes automatic loss of Belgian citizenship under Belgian law or not.

Now, what the US has say over is whether you have US citizenship, and so perhaps part of your question is whether getting a Belgian passport affects your US citizenship. The only ways it can affect your US citizenship are 1) if getting a foreign passport, or otherwise exercising the rights of a foreign nationality, causes loss of US citizenship, or 2) if failing to renounce your existing nationalities before or a certain time after your US naturalization, or continuing to exercise the rights of those nationalities, makes your naturalization fraudulent, which would mean you never had US citizenship.

Number 1 is not true because it is unconstitutional for a properly naturalized (i.e. not fraudulently) US citizen to lose US citizenship under any circumstances unless (it is proven that) the person personally intended to relinquish US citizenship. There are certain expatriating acts but it always requires intent to relinquish US citizenship in order to lose it.

Number 2 is also not true because something can only make your naturalization fraudulent if it violates a requirement of naturalization at the time of naturalization, but the US naturalization process has never required an applicant to have taken an action to renounce their existing nationalities, or to promise to take an action in the future to renounce those nationalities. Sometimes people mention the oath, but the oath never actually mentions renouncing citizenship or nationality; instead it just says renounce "allegiance and fidelity", which are just abstract concepts in the mind. (Also note that "renounce" is in the present tense, which would contradict an affirmation of having taken some action in the past, which would be past tense, or a promise of taking an action in the future, which would be present tense. Only renunciation of something abstract in the mind, which requires no action, and be done in the "present" at the time of the oath.) No official source has interpreted the oath to mean an affirmation of having taken some action, or a promise to take some action, regarding the person's existing nationalities; to the contrary, many official sources talk of the possibility of continued existence of multiple nationality after naturalization. Therefore, not renouncing Belgian citizenship could not possibly make your naturalization fraudulent.