Oral cancer represents a worldwide public health problem, being among the most prevalent, associated with high morbidity and mortality rates. This systematic review aimed to review the causes of the delayed diagnosis of oral cancer mainly in the elderly, in developed and developing countries. Search strategy was developed for MEDLINE databases (via PubMed), EMBASE, Web of Science, SCOPUS, and LILACS and for grey literature (Google Scholar, ProQuest and OpenGrey), without language or period restrictions. The risk of bias was assessed using instruments from the Joanna Briggs Institute and the quality of evidence according to the GRADE system. The search resulted in 14,473 records, of which only 13 met the eligibility criteria. The total sample was 1,705 participants, with a predominance of males. All studies included reported causes of delayed diagnosis of oral cancer related to the patient and five also reported causes related to health professionals. The scarce knowledge of the population was pointed out as the main cause of delayed diagnosis of this cancer. Regarding the risk of bias, ten studies were classified as low risk and three, as moderate risk. The quality of the evidence was very low for the outcome related to delayed diagnosis of oral cancer. Wide dissemination of information on oral cancer is needed, especially for the elderly, such as its initial signs and symptoms, in developed and developing countries. Further studies should be conducted to better understand the causes of delayed diagnosis of oral cancer in countries with different socioeconomic statuses.