Bhagavaa is the nom./voc. singular form of Bhagavant, the Fortunate
One, an epithet for the Buddha.
DN22 Mahasatipatthana Sutta CSCD372/PTS2.289
Tatra kho bhagavaa bhikkhuu aamantesi - "bhikkhavo"ti.
there / the Bhagavant / monks / addressed / monks / (end-quote)
There, the Buddha addressed the monks: "O monks."