The terms third order (secular )and oblate refer to a lay person who is a member of a religious community usually without taking vows. The mendicant orders such as Franciscans, Dominicans, Carmelites, Servites, Augustinians,Mercedarians,and Trinitarians have lay people who are part of their order called third order members. Third order members belong to the entire religious order.Third order members today are often called Secular Franciscans or Secular Carmelites, for example. Different name, same commitment.
Oblates, on the other hand, do not belong to an entire religious order but to a specific religious house, usually a monastery of contemplative monks or nuns. Most Oblates are Benedictine, but the Handmaids of the Precious Blood in Jemez Springs, NM, also have oblates.
Other religious institutes, mostly newer ones, have lay associates or confraternities.Third orders, oblates, lay associates, and confraternities all are ways for a lay person to belong to a religious family by sharing in its charism, but without taking vows like the religious members of the community do. The lay people are nonetheless still a part of the particular religious family, and these forms of commitment are encouraged by the Church.
