Hi, corn oil is the vehicle for tamoxifen administration, however this takes a long time, with vortex/shaking and with heated temperatures (see attached article). Hence, usually people will include a corn oil (without tamoxifen) group as their vehicle controls.
Article Tamoxifen Administration to Mice
Because tamoxifen is poorly soluble in aqueous buffers, it can be made more soluble by first dissolving in ethanol, before further dilutions in corn oil.