Ostmanntūn is Difelin underburg, on Īrlande. Ostmanntūn stent on ceastre norþhealfe betwuh ēa and Smiþfeld Cēapstōwe.

Þa Heanrig II Engla cyning cwōm on Difeline, manig þāra Ostmannum, þā bōgedon Difelinrīce, flīemedon ofer þǣre ēa of Difelinbyrig and timbroden nīw tūn on ēa norþhealfe sē is Ostmanntūn.

On Īrisc is his nama Baile Lochlannach se is "Norþmanna burg".