Oss has a typical Dutch maritime climate: mild, wet, and unpredictable. Summers are warm rather than hot, with average highs around 22°C (72°F) in July and August. Winters are grey and damp but rarely severe — temperatures below freezing are possible but prolonged cold snaps are unusual.
Spring (April to June) is the most pleasant time to visit: days are lengthening, the countryside is green, and the tourist crowds that descend on Amsterdam are nowhere in evidence. Autumn is similarly agreeable. Summer weekends can be lively in the Grote Markt, but Oss never gets oppressively busy.
Carnival is the single best reason to time a visit. Oss takes its pre-Lenten celebrations seriously — in Noord-Brabant, Carnival is a bigger event than Christmas, and Oss is one of the more enthusiastic participants. The festivities run for three days before Ash Wednesday (usually late February or early March), with floats, costumes, and music on the Grote Markt and surrounding streets. If you come during Carnival, book accommodation well in advance.