The purpose of this study was to investigate the students’ performance improvement in the fifth grade of a primary school in Greece in the subjects of Language, Mathematics, Science, Geography and History with the support of open educational resources (OER). A quasi-experimental approach was used for the data collection due to the inability to collect data from a random sample. Sixteen students from a fifth-grade class participated in the experimental group and sixteen from other fifth-grade glass class in the control group. Both groups were given pre-tests and post-tests. The quantitative data were analyzed using descriptive statistics, Schapiro-Wilk's normality test, Levene's test for equality of variances and covariance analysis (ANCOVA). The results of data analysis showed that the post-test students' performance in Language, Mathematics and Science subjects was statistically significant with ANCOVA. On the contrary, the performances in Geography and History seem to be statistically not significant.