In 3.0, both druid and shaman used the same system for obtaining an animal companion, based on the animal friendship spell.
In 3.5, the druid’s system for obtaining an animal companion was overhauled, and animal friendship ceased to be a spell. And while the shaman’s animal companion goes unmentioned in the Oriental Adventures 3.5 update, its spellcasting is addressed:
The 3.5 revision introduced many changes to spells. Refer to the 3.5 Accessory Update (available at www.wizards.com) for general information about changes to spell names, levels, and schools. Player’s Handbook should change the same way for Oriental Adventures spellcasters. For example, animate dead used to be a 5th-level wizard spell (and a 5th-level maho-tsukai spell); now it is 4th level for both wizards and maho-tsukai.
Thus, the animal friendship spell is no longer a shaman spell, since it is no longer a spell at all. The shaman animal companion feature therefore no longer functions as written.
In a 3.5 game, I strongly feel the only sensible interpretation of these facts is that the Dragon magazine article assumed that it was understood that the shaman’s animal companion class feature would be updated as was the druid class feature of the same name. As always in any printed medium, space in Dragon magazine was at a dire premium, and they probably saved quite a lot of it by not specifying each class feature that mirrors a core class’s feature is updated the same as that core class. But this comes at the cost of some confusion for us, particularly since the shaman’s animal companion class feature doesn’t actually reference the druid at any point, and it’s not immediately obvious to someone unfamiliar with the 3.0 druid that it is the same thing.
Bottom line: in a 3.0 game, the shaman’s animal companion feature works like the druid’s. The same should also be true in a 3.5 game, so the shaman should use the 3.5 druid’s animal companion rules.