Chlorophyll doubles Oddish's Speed, giving it a Speed stat of 26, which outspeeds all but a handful of Choice Scarf users, namely Mienfoo and Chinchou. 0 HP IVs give Oddish 19 HP, which means Oddish only takes 1 damage from Life Orb recoil. Solar Beam hits in one turn because of the sun, and thanks to STAB, it has a massive power of 180. This will hit any Pokemon that doesn't resist it extremely hard, except for a select few special sponges such as Lickitung and Munchlax. Hidden Power Fire is Oddish's secondary attack, and gets a 1.5x boost from the sun, giving it a power of 105. This is Oddish's only attack that hits Steel-type Pokemon super effectively, as they resist Solar Beam and are immune to Sludge Bomb. It is also useful against Grass-type Pokemon, as many of them have Poison as their secondary type, namely Bulbasaur, Bellsprout, Oddish, and Budew. Sludge Bomb is Oddish's second most powerful attack other than Solar Beam. Coverage-wise, it hits Fire- and Dragon-types, and it can be used outside of sun. Sleep Powder is a great alternative to Sludge Bomb, allowing Oddish to put a member of the opponent's team to sleep and then take free shots at them. Sleep Powder's prime use is to put the opponents sun counters to sleep, should they carry any.
Attacking-wise, there isn't much else that Oddish can run on the set, as the moves listed are its best options. However, there are a few alternatives that you can run in the last slot. Stun Spore can be used to cut the Speed of faster Pokemon. Synthesis can be used for reliable recovery in the sun. When equipped with an Eviolite, Oddish takes on the role of a bulky sweeper. If you decide to run an Eviolite, an EV spread of 156 HP / 236 SpA / 116 Spe with a Modest nature is recommended. This spread gives Oddish added bulk while maximizing its Special Attack. You should also make use of Synthesis for recovery if you choose this spread.
Stealth Rock support is appreciated to weaken Fire- and Flying-type Pokemon, giving Oddish an easier time sweeping against Pokemon such as Houndour. Stealth Rock is also useful for beating Snover, as it takes 25% every time it comes in to induce hail. Fighting-types, such as Mienfoo and Croagunk, are good partners because they can counter Munchlax and Lickitung, who would otherwise sponge Oddish's special attacks with ease. Wynaut makes a great teammate as well, since it has the ability to trap Snover, which is the biggest counter to sun sweepers.