We use the conformal bootstrap approach to explore 5D CFTs with O(N ) global symmetry, which contain N scalars φ i transforming as O(N ) vector. Specifically, we study multiple four-point correlators of the leading O(N ) vector φ i and the O(N ) singlet σ. The crossing symmetry of the four-point functions and the unitarity condition provide nontrivial constraints on the scaling dimensions (∆ φ , ∆ σ ) of φ i and σ. With reasonable assumptions on the gaps between scaling dimensions of φ i (σ) and the next O(N ) vector (singlet) scalar, we are able to isolate the scaling dimensions (∆ φ , ∆ σ ) in small islands. In particular, for large N = 500, the isolated region is highly consistent with the result obtained from large N expansion. We also study the interacting O(N ) CFTs for 1 N 100. Isolated regions on (∆ φ , ∆ σ ) plane are obtained using conformal bootstrap program with lower order of derivatives Λ; however, they disappear after increasing Λ. We think these islands are corresponding to interacting but nonunitary O(N ) CFTs. Our results provide a lower bound on the critical value N c > 100, below which the interacting O(N ) CFTs turn into nonunitary. The critical value is unexpectedly large comparing with previous estimations.