BD-Oligo is a promising fluorescent sensor designed for detecting Aβ oligomers. It effectively monitors the dynamics of these oligomers during fibril formation in vitro and shows potential for in vivo staining in Alzheimer's disease (AD) mouse models. The sensing mechanism is facilitated through π-π stacking interactions and hydrogen bonding between BD-Oligo and the exposed hydrophobic regions of Aβ oligomers, as confirmed by computational techniques. Furthermore, BD-Oligo can cross the blood-brain barrier (BBB) to detect oligomers in the brains of AD transgenic mice without causing toxicity.