Very low birth weight (VLBW) infants, mostly preterm, have many barriers to feeding directly from the mother's breast, and need to be fed alternatively. Feeding is a major influencer in oral microbial colonization, and this colonization in early life is crucial for the promotion of human health. Therefore, this research aimed to observe the establishment of oral microbiome in VLBW infants during their first month of life through hospitalization, and to verify the impact caused by the implementation of oral diet on the colonization of these newborns. We included 23 newborns followed during hospitalization and analyzed saliva samples collected weekly, using 16S rRNA gene sequencing. We observed a significant decrease in richness and diversity and an increase in dominance over time (q-value < 0.05). The oral microbiome is highly dynamic during the first weeks of life, and beta diversity suggests a microbial succession in early life. The introduction of oral diet does not change the community structure, but affects the abundance, especially of Streptococcus. Our results indicate that although time is related to significant changes in the oral microbial profile, oral feeding benefits genera that will remain colonizers throughout the host's life.