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Four multiparous dairy cows were used in a 4 × 4 Latin square to examine how starch level and oil mixture impact dry matter (DM) intake and digestibility, milk yield and composition, rumen fermentation, ruminal methane (CH₄) emissions, and microbial diversity. Experimental treatments comprised high (HS) or low (LS) levels of starch containing 0 or 30 g of a mixture of sunflower and fish oils (2:1 <i>w</i>/<i>w</i>) per kg diet DM (LSO and HSO, respectively). Intake of DM did not differ between cows fed LS and HS diets while oil supplementation reduced DM intake. Dietary treatments did not affect milk and energy corrected milk yields. There was a tendency to have a lower milk fat concentration due to HSO compared with other treatments. Both high starch level and oil supplementation increased digestibility of gross energy. Cows receiving HS diets had higher levels of total rumen VFA while acetate was lower than LS without any differences in rumen pH, or ruminal CH₄ emissions. Although dietary oil supplementation had no impact on rumen fermentation, decreased CH₄ emissions (g/day and g/kg milk) were observed with a concomitant increase in <i>Anoplodinium-Diplodinium</i> sp. and <i>Epidinium</i> sp. but a decrease in <i>Christensenellaceae</i>, <i>Ruminococcus</i> sp., <i>Methanobrevibacter ruminantium</i> and <i>Mbb. gottschalkii</i> clades.