This paper analyzes the two types of complement clauses in the Uto-Aztecan language O'dam (Southeastern Tepehuan), Controlled and Non-Controlled complements. These complements are analyzed within the framework of Lexical Functional Grammar (Kaplan & Bresnan 1982). This paper argues that Controlled complements are best analyzed as COMP (i.e., clausal) arguments of their matrix verb, while Non-Controlled complements are OBJ (i.e., nominal) arguments of the matrix clause. For Non-Controlled complements the OBJ is a pronoun referring only to the eventuality of complement clause, while the other dependents of the clause are introduced via an adjunct headless relative clause. Three pieces of evidence are used in favor of this analysis: i) the similar syntactic shapes of Non-Controlled complements and headless relative clauses, ii) the interpretive distinction between a Non-Controlled complement and a DP complement, and iii) the result of the preverbal quantification argumenthood test.