Otsaju ehk telentsefalon (ladina telencephalon ) on kesknärvisüsteemiga loomadel embrüonaalses arengujärgus ajuosa, mis moodustub prosentsefalonist.
Koos vaheajuga on ta eesaju (prosencephalon) osa.
Otsaju on sekundaarsetest ajupõitest esimene, sellest areneb suuraju.
Madude embrüotel on otsaju tuvastatav üsna kohe peale munade väljutamist.
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