An experiment was conducted to determine whether adding oat hulls to weaner pig diets based on extruded rice or unprocessed wheat influenced post-weaning diarrhoea (PWD) and protein fermentation in the large intestine. Ninety-six male piglets (5.16 (SEM 0.08) kg) were allocated to (i) extruded rice plus animal proteins (RAP); (ii) RAP with added oat hulls (20 g/kg); (iii) wheat plus animal proteins (WAP); (iv) WAP with added oat hulls (20 g/kg). Blood and faecal samples were collected on days 7 and 14 after weaning at about age 21 d. Pigs fed RAP had more PWD than pigs fed WAP (P < 0.05). Oat hull supplementation to diet RAP decreased the incidence of PWD (P < 0.05). The total-tract digestibility of DM, starch and energy was higher in rice-based diets than in wheat-based diets (P < 0.001); however, oat hulls decreased digestibility of DM and gross energy (P < 0.001). Pigs fed RAP had higher plasma creatinine concentrations (P < 0.01), which were positively correlated to cumulative beta-haemolytic Escherichia coli scores after weaning (R2 0.928; P = 0.015). Addition of oat hulls decreased plasma urea concentrations only in pigs fed RAP (interaction; P < 0.05). Pigs fed RAP had lower faecal total biogenic amine concentrations than pigs fed WAP (P < 0.001). Oat hull supplementation tended to decrease total biogenic amine concentrations (P = 0.103). These data indirectly suggest that a mostly insoluble dietary fibre source such as oat hulls can decrease PWD in dietary situations where there may be a misbalance of carbohydrate to protein entering the hindgut.