The OMX Stockholm 30 is a stock market index for the 30 largest and most traded stocks on the Nasdaq Stockholm stock exchange. It is a price return and modified free float capitalization-weighted index. The OMXS30 index started on 30 September 1986 with a base value of 125 and it is reconstituted and rebalanced semi-annually on the first trading day in January and July. The original name of the index was just OMX, but it was changed to OMXS30 on 15 November 2004. On 1 July 2025, Nasdaq changed the methodology for the OMXS30 index. Previously the criteria for inclusion of a company's stock in the index was based on liquidity. After the change the primary criterion for inclusion is the free float market capitalization of the company. To still ensure good liquidity there is a minimum liquidity requirement of 50 million SEK in average daily value traded over a six month period. The updated methodology only allows one share class to be included in the index and no security can exceed 15 percent of the index. The constituent weighting was also changed to a free float adjusted market capitalization weighting. Wikipedia