– 6 – so far have focused on the modeling of individual objects such as TW Hydra, (Calvet et al. 2004), GM Aur and DM Tau (Calvet et al. 2005), LkH α 330 (Brown et al. 2008), and LkCa 15 (Espaillat et al. 2008). To date, few studies have discussed the properties of transition disks as a group (e.g., Najita et al. 2007; Cieza et al. 2008). These papers studied relatively small samples, suffer from different selection biases, and arrived, not too surprisingly, at very different conclusions. Najita et al. (2007) studied a sample of 12 transition objects in Taurus and found that they have stellar accretion rates ∼ 10 times lower and a median disk mass ( ∼ 25 M$_{JUP}$ ) that is ∼ 4 times larger than the rest of the disks in Taurus. They argue that most of the transition disks in their sample are consistent with the planet formation scenario. The disk masses found by Najita et al. are in stark contrast to the results from the SMA study of 26 transition disks from Cieza et al. (2008). They observed mostly WTTSs disks and found that all of them have very low masses < 1-3 M$_{JUP}$ suggesting that their inner holes were more likely due to photoevaporation, instead of the formation of jovian planets. This discrepancy can probably be traced back to the different sample selection criteria as Najita et al. studied mostly CTTSs, while Cieza et al. studied mostly WTTSs (see § 5.1.1). A much larger and unbiased sample of transition disks is needed to quantify the importance of multiplicity, photoevaporation, grain growth, and planet formation on the evolution of circumstellar disks. This paper is the first part of a series from an ongoing project aiming to characterize over 100 Spitzer -selected transition disks located in nearby star-forming regions. Here we present millimeter wavelength photometry (from the SMA and the CSO), high-resolution optical spectroscopy (from the Clay, CFHT, and Du Pont telescopes), and Adaptive Optics near-infrared imaging (from the VLT) for a sample of 26 Spitzer -selected transition circumstellar disks located in the Ophiuchus molecular cloud. We use these new ground- based data to estimate the disk mass, accretion rate, and multiplicity for each object in our sample in order to investigate the mechanisms potentially responsible for their inner