Testaccio - moderate reqs for lunch or light dinner
I second Maureen on Volpetti Piu.
I think she means Rosticerri', which is excellent. Owned by the same person as the fancy La Rosetta, it is a tavola calda, so you can certainly eat light (but not necessarily cheap)
Another 'snack' place would be 00100, which serves pizza by the slice as well as 'tramezzini', triangles of pizza stuffed with things like coda alla vacinara, etc.
My one experience eating at Oasi della Birra was pretty bad, though I have bought wine there happily for decades. I like Divinare, wine bar, on via Aldo Manuzio, and Volpetti Più, tavola calda of the Volpetti shop. Remo is good, I think always crowded. There is another high-end tavola calda with some seating (not tables) on Piazza Testaccio, RosticceRè.