Organoids are powerful models of brain development and function. Mouse brain organoids reproduce the main steps of neurodevelopment in vivo and differentiate more rapidly than human organoids, making them scalable for screening applications. However, mouse brain organoids remain poorly defined. Here, we showed that the transcriptome of mouse brain organoids developed for three weeks is close to that of a neonatal mouse brain. Strikingly, organoids reproduced the majority of alternative splicing and polyadenylation site events that define a neural identity in vivo. Proteomics revealed that most changes in mRNA expression were translated into proteins. Mouse brain organoids differentiated for only three weeks already harboured a complex network of synaptic proteins, including the ionotropic and metabotropic receptors for the neurotransmitters glutamate and GABA. To conclude, our study provides compelling evidence that mouse brain organoids are a rapidly maturing and relevant model of mammalian brain development and function in a neonatal-like environment.