The timing of plant developmental transitions is decisive for reproductive success and thus tightly regulated by a number of pathways with a high degree of crosstalk between them. Such complex regulatory pathways often involve post-translational modifications (PTMs), integrating internal and environmental signals. O-glycosylation, the attachment of a single monosaccharide to serine or threonine of nuclear and cytosolic proteins, is one of these PTMs, affecting a number of very diverse proteins. Here we show that mutants in the Ofucosyltransferase SPINDLY (SPY) show accelerated developmental transitions. In plants, the transition from juvenile to adult and later to reproductive phase is controlled by an endogenous pathway regulated by miR156, targeting the SQUAMOSA PROMOTER BINDING PROTEIN (SBP/SPL) family of transcription factors. SPLs regulate a number of developmental processes, such as trichome formation, leaf shape, leaf growth rate and floral transition. We present genetic analysis showing that O-glycosylation regulates transitions independently of miR156 levels, but depending on functional SPLs. Moreover, SPLs interact directly with SPY and are O-glycosylated. Our results suggest a model where O-glycosylation is involved at several steps in the regulation of developmental transitions, and plays an important role in fine-tuning different regulatory pathways.