Er and Oh

Er noun - A trivalent metallic element of the rare earth group; occurs with yttrium.

Oh and er are semantically related in emphatic exclamation topic. In some cases you can use "Oh" instead a noun "Er".

Oh noun - The numerical symbol 0 or the absence of number or quantity represented by it.

Er and oh are semantically related in boo topic. You can use "Er" instead a noun "Oh", if it concerns topics such as wow, emphatic exclamation.