Uprising ground-level ozone (O₃) and its regional pollution in eastern China are attracting more attention. On top of local precursor emissions and photochemistry, background ozone and long-range transport also contribute significantly to O₃ concentrations. To quantify the regional O₃ background concentrations and its yearly and seasonal variations, multiple methods, including the principal component analysis (PCA) and the Texas Commission on Environmental Quality (TCEQ) method, were applied for a case study in Shandong (SD) province in North China, where serious O₃ pollution occurred frequently yet the background contributions have not been well quantified. Results derived from multiple methods show an overall consistent trend with 2018-2020 averaged regional background O₃ (MDA8) of 88.9 μg/m³, accounting for 79.4% of total O₃ in the region. From 2018-2020, the changes of regional MDA8 O₃ estimated by Methods 1, 2, 3, and 4 are -3.8 μg/m³,1.6 μg/m³, -5.2 μg/m³ and 0.9 μg/m³, respectively. Clear seasonal variations in the regional background O₃ are observed, showing a pattern of summer > spring > autumn > winter. In addition, the regional ozone contribution at coastal cities was larger than that for inland cities whereas local O₃ formation gradually increased from coastal areas to inland areas. The sea-land wind contribution to O₃ in the eastern coastal cities in summer was around 2.1% at the three-year average level, while the local photochemistry to O₃ in the inland cities was about 29.7% during ozone season. Local photochemical contribution to O₃ in inland cities during ozone pollution episodes can reach up to 55.8%.