I have no idea what Oddish's deal is since nobody has talked about it in this thread or the previous one. Oddish can drop for all I care.
Not really opposed to Oddish's dropping, but I'd be happy to discuss why it's in B right now. Simply put, it's because it turns into a very good lategame Grass type due to the (late) Petal Dance and a high special attack stat, generally getting to that point just around the Sootopolis gym, and prior to that a fairly strong support movepool in Sleep Powder, backed up by its evolution's fairly high defenses.
Oddish's main issue is that its offensive level up movepool is barren, being Acid right around evolution and Absorb before that. Its TM movepool does help somewhat, with an option for Bullet Seed to slightly beef up its grass STAB until Giga Drain is available, as well as Secret Power not long after obtaining it, something not heavily contested and which Gloom makes surprisingly decent use of due to its passable attack stat. Sludge Bomb is similar, providing a reliable STAB that Vileplume can make fine use of with its decent 80 base attack.
In terms of major battles, Oddish takes on Rival 2's Wailmer and Marshtomp well due to type advantage, can take on Lombre and Shroomish but will take a long time in the process, and does poorly against everything else. Oddish does fairly well against Wattson's non-Magnemite/ton Pokemon in spite of its low attacking power options, due to a resistance to their STAB and Sleep Powder providing a large amount of breathing room. Selfdestruct from Voltorb is incredibly painful of course. Oddish doesn't do particularly well against Team Magma and Aqua due to the massive amount of Zubat they use, although it can usually take on the Poochyena and Carvanha lines, especially later on when its movepool opens up.
Flannery is a no go for obvious reasons. The by this point Gloom actually does alright against Norman, being capable of taking on at least one of his Pokemon because of Sleep Powder's incredible utility. Rival 3 Gloom (or even Vileplume at this point) excels against, taking on Wailmer, Shroomish, Lombre, Marshtomp, and Grovyle. As Vileplume it is possible to take on the Pelipper in Emerald, but it's a difficult fight that relies too much on Sleep Powder to be comfortable. Winona is also a bad time for Gloom/Vileplume, although with the Sludge Bomb TM you could probably take on the Emerald Tropius.
Tate and Liza are interesting but not particularly favorable to Vileplume, particularly in Emerald due to Xatu. Vileplume has likely been able to feed heavily off the water route trainers at this point, so it's not a stretch to have Vileplume take on Claydol, Solrock, or Lunatone with Giga Drain, but it's not a gym where Vileplume will feast like it does for Sootopolis. Vileplume should be fully capable of taking on the Sootopolis gym, as long as you've given either the Giga Drain TM or if you've just gotten to Petal Dance. Sealeo can pause Vileplume temporarily while Milotic and Kingdra usually call for Sleep Powder, but otherwise Vileplume rolls over the gym.
The Elite Four depend somewhat on whether or not you've given Vileplume Sludge Bomb. Vileplume can reasonably take on (requires Sludge Bomb in brackets) -
Sidney: Sharpedo/Crawdaunt, generally Mightyena, [Cactune, Shiftry]
Phoebe: Banette 1, generally Dusclops 1, Sableye (Emerald)
Glacia: Sealeo 1 (Sleep Powder needs not to miss and let Sealeo build up Ice Ball)
Drake: maybe Kingdra, [Shelgon, Altaria]
Steven: Claydol, generally Cradily (Sludge Bomb helps a lot)
Wallace: Whiscash, Ludicolo (even Acid lets you take it on), generally Wailord, generally Milotic
Oddish comes into its niche early after catching it, namely its role as a rather bulky and strong sleep inducer, and lays heavy claim to a role as a grass type attacker with appropriate TM support, requiring little in the way of investment beyond that.