When substituting an essential oil purely for aromatic blends and not therapeutic emotional or physical purpose you can select an oil from the same family of oils (i.e. citrus, floral, spicy, earthy, etc.) as the oil you are substituting for.
For instance, if you need a substitute for the essential oil of mandarin, the essential oil of sweet orange will provide a similar aroma. If you need to substitute for rose otto (steam distilled rose essential oil), rose geranium or rosewood (bois de rose) is not perfect, but it can give a similar result.
Below are a few oils that will give you examples for aromatic substitutions. Even for aromatic blending, you still need to be aware and follow all safety precautions for the oils that you choose.
Lemon - Grapefruit
Tangerine - Orange - Mandarin
Peppermint - Spearmint
Bergamot - Grapefruit
Melissa - Lemon
Aniseed - Fennel
Cinnamon - Clove