OO Serpentis is a deeply embedded young star which brightened by 5 mag in Kband between 1994 and 1995, and gradually faded afterwards. In many respects the star and its outburst is very similar to the well-known young eruptive stars (FUors and EXors). We monitored the eruption of OO Ser for 20 months with the Infrared Space Observatory in the 3.6-100 μm wavelength range. We complemented this dataset with new (ground-based and spaceborn) infrared observations from 2004 and constructed the light curve of the outburst at 10 different wavelengths. Our results show that by 2004 OO Ser already returned to the pre-outburst state at 2.2 μm, suggesing a significantly shorter outburst timescale than that of classical FUors, but longer than that of EXors. This may indicate that OO Ser -- together with the recently erupted star V1647 Ori -- is the first example of an intermediate object between FUors and EXors.