Provided herein, among other things, are compositions for improving signal-to-noise in hybridization to oligonucleotide arrays. In some embodiments, the composition may comprise a population of indexing oligonucleotides of formula X-Y, where the sequences of region X hybridize to an array and the sequences of region Y hybridize to distinct sites within a genomic region, transcript, set of transcripts, or complement of the same. Use of these oligonucleotides increases signal, reduces background and smoothens array hybridization analyses. Methods and kits that employ the compositions are also provided.