After taking enough steps, the Egg will hatch into a level 5 (Generation II and III) or level 1 (Generation IV onward) first-stage Pokémon of the female species's (or non-Ditto parent's) evolutionary chain.
Since Oddish is the female Pokémon, the egg will hatch into Oddish.
If you want a Bulbasaur, breed the current one you have with a Ditto. You can also find a Female Bulbasaur and breed it with a Male Pokémon in the Grass or Monster Egg Group or a Ditto.
Source: https://m.bulbapedia.bulbagarden.net/wiki/Pokémon_breeding
