Involved in ribosomal large subunit assembly, required for normal progression of rRNA processing (PubMed:26268215, PubMed:29375609). S-adenosyl-L-methionine-dependent methyltransferase that probably methylates the C(5) position of cytosine 2268 (m5C2268) in nuclear 25S rRNA (PubMed:26268215). May play a role in the regulation of the cell cycle and the increased nucleolar activity that is associated with the cell proliferation (By similarity). Seems involved in the regulation of cell proliferation in leaves (PubMed:19392710, PubMed:29375609). a cytidine in 25S rRNA + S-adenosyl-L-methionine = a 5-methylcytidine in 25S rRNA + H(+) + S-adenosyl-L-homocysteine Strongly expressed in tissues with high cell proliferation activity that have a high demand for ribosome production such as guard cells, leaves primordia, root apical meristems and the basal parts of lateral roots. Moderate reduction in leaf cell number associated with pointed leaves shape (PubMed:19392710). Defects in pre-rRNA processing characterized by an increased accumulation of rRNA intermediates containing 50-ETS, ITS1, or ITS2, with stronger negative effect on ITS2-containing intermediates (PubMed:29375609). Higher levels of 35S, 27SA, 27SB, P-A3, and 18SA3 rRNAs (PubMed:29375609). Normal levels of methylation at cytosine 2860 of 25S rRNA, but slight reduction of nuclear 25S rRNA cytosine 2268 (m5C2268) (PubMed:26268215). Double mutants oli2 oli7 and oli2 oli5 have further reduced cell number but exhibit also excessive postmitotic cell enlargement in leaves (compensation phenotype) (PubMed:19392710). Plant missing both OLI2 and GIF1/AN3 have a strong compensation phenotype (PubMed:19392710). The double mutant gdp1 oli2 exhibit strong growth defect due to a synergistically impaired cell proliferation in leaves and enlarged cells (PubMed:29375609). Belongs to the class I-like SAM-binding methyltransferase superfamily. RsmB/NOP family. Truncated N-terminus.