As with all liquids, transformer oil also expands and contracts depending on the temperature. In large capacity transformers, this is more prominent. When the temperature becomes more, the oil expands into the conservator. Large transformers typically have a "balloon" arrangement (also called as COPS), so when the oil rises, the air in the balloon exhales via the breather. Conversely, when the oil contracts, the balloon expands drawing in the air from the breather. There are also small capacity transformers which do not have conservators, but usually their capacities do not exceed 2.5 MVA. Hope this helps.