QUITO, Ecuador—International election observers didn't find evidence of fraud in Ecuador’s presidential vote, the Organization of American States said Monday, as the group congratulated the ruling party’s candidate and urged conservative challenger Guillermo Lasso to pursue any appeal to electoral authorities.

Lenin Moreno, Ecuador’s leftist former vice president, had 51.17% support compared with 48.83% for Mr. Lasso, with 99.5% of the votes counted from Sunday’s vote, according to the national electoral council. The body...