This paper investigates the effects of rainbow gravity on the spectroscopic structure of Klein-Gordon (KG) oscillators in a mixed magnetic field in the context of cosmic string spacetime. The mixed magnetic field is described by the 4-vector potential $ A_{\mu} = (0, 0, A_{\varphi}, 0) $, where $ A_{\varphi} = B_{1} r^{2}/2 + B_{2} r $, with $ B_{1} $ and $ B_{2} $ representing the strengths of uniform and non-uniform magnetic fields, respectively. The study uses a loop quantum gravity motivated rainbow function, which leads to interesting effects when $ B_{2} $ increases, including energy level crossings that flip the spectra of the KG oscillators upside down. Additionally, Landau-like signatures are observed on the spectra, and the magnetic field is found to become energy-dependent under rainbow gravity. The paper also introduces a new conditionally exact solution for the KG oscillators, truncating the biconfluent Heun functions into polynomials. The results show that rainbow gravity affects both the spectroscopic structure and the magnetic field, with the maximum energy remaining below the Planck energy scale. The study highlights the importance of rainbow gravity in modifying the energy levels of quantum particles and the magnetic field structure, providing a framework for understanding the effects of quantum gravity on particle dynamics in mixed magnetic fields. The findings are supported by numerical plots and analytical derivations, demonstrating the impact of rainbow gravity on the behavior of KG oscillators in cosmic string spacetime.This paper investigates the effects of rainbow gravity on the spectroscopic structure of Klein-Gordon (KG) oscillators in a mixed magnetic field in the context of cosmic string spacetime. The mixed magnetic field is described by the 4-vector potential $ A_{\mu} = (0, 0, A_{\varphi}, 0) $, where $ A_{\varphi} = B_{1} r^{2}/2 + B_{2} r $, with $ B_{1} $ and $ B_{2} $ representing the strengths of uniform and non-uniform magnetic fields, respectively. The study uses a loop quantum gravity motivated rainbow function, which leads to interesting effects when $ B_{2} $ increases, including energy level crossings that flip the spectra of the KG oscillators upside down. Additionally, Landau-like signatures are observed on the spectra, and the magnetic field is found to become energy-dependent under rainbow gravity. The paper also introduces a new conditionally exact solution for the KG oscillators, truncating the biconfluent Heun functions into polynomials. The results show that rainbow gravity affects both the spectroscopic structure and the magnetic field, with the maximum energy remaining below the Planck energy scale. The study highlights the importance of rainbow gravity in modifying the energy levels of quantum particles and the magnetic field structure, providing a framework for understanding the effects of quantum gravity on particle dynamics in mixed magnetic fields. The findings are supported by numerical plots and analytical derivations, demonstrating the impact of rainbow gravity on the behavior of KG oscillators in cosmic string spacetime.