Channel Charting (CC) is a promising tool for managing radio resources, especially those tied to user location. Conventionally, CC involves a dimensionality reduction of channel state information (CSI) to obtain a low-dimensional latent space. While conceived to be performed at the network side (e.g., base stations or access points), CC could be beneficial for end-devices. In practice, end-user devices often have access to heterogeneous and non-cooperating networks, with no coordination. In such a setup, decision-making must occur on the user side to fully leverage available networks. For this reason, user-side CC is particularly relevant. In this work, we introduce a user-side CC framework: Opportunistic Channel Charting (OCC). OCC allows mobile devices to localize themselves in the radio environment opportunistically. Unlike CC, OCC allows devices to build a chart from passively acquired CSI. This is done by monitoring the ambient wireless channels. Via real-world experiments, using commodity devices, we validate OCC. Our results show that OCC captures geometric information comparable to conventional CC in terms of trustworthiness and continuity while requiring no cooperation from surrounding devices. During measurements of CSI, we also measure network quality. To exploit this, we propose a new OCC-based Wi-Fi Access Point (AP) selection (OCCAPS).