Size comparison with a cigarette pack

Underside

This kit includes the Hazel Kai's backpack,
and while the sculpt is similar to the HGUC Hazel Kai, it uses stickers instead of color separation for certain areas. (the yellow parts)

The winch cannon mount position can be adjusted with a sliding mechanism.

The winch cannon can be opened by removing the cover and sliding it to the rear before plugging it in.

Various parts included to build the Hazel Owsla.

On top of that, you'll need to gather parts from four other kits.
1. Hazel Kai – lower body, arms, head, weapons
2. Advanced Hazel – feet, sub-arm, head unit
3. Hazel Unit 2 – Tri-Booster backpack
4. Hrududu – Long Blade Rifle

After disassembling the core block as shown above,

remove the small wing from the shoulder unit.

The right shoulder gets the wing unit swapped out for the Owsla version,
while the left shoulder has its existing wing unit detached and repositioned slightly higher before reattaching.

Once you attach the additional waist and neck parts, the torso is complete.

Attach the Hazel Owsla shoulder parts onto the existing Hazel Kai arms.

Position the winch cannon's sub-arm at the angle shown above, then

arrange and connect everything as shown to complete the combination.

At this point, the sub-arm on top of the right shoulder can be plugged into the inner pin at the rear of the Long Blade Rifle to lock it in place.

Remove a number of parts from the Hazel Owsla as shown above.

You can use the backpack included with Primrose,
but since it doesn't come with a beam saber, you'll need to borrow one from the HGUC Hazel Kai.
(At that point, you might as well just borrow the whole Hazel Kai backpack since it has better color separation anyway.)

Comparison shot of the HGUC Hazel Kai, Next-Generation Mass Production Type, and Owsla.
If you have 3 Hazel Kais, 2 Advanced Hazels, and 2 Primroses, you can display them like this (...)

Together with the separately sold HGUC Hrududu.

First, disassemble the Hrududu as shown above, then

set the shield mounting gimmick on both sides of the nose sensor unit into a V-shape as shown.

Rotate the nose sensor unit to face rearward and attach it — now it's ready.

For Primrose, remove the winch cannon and shoulder armor,
then plug the joint parts included with the HGUC Hrududu onto the top of the backpack and attach the prepared Hrududu.

Underside

You'll need the Tri-Booster from the HGUC Hazel Unit 2 and two winch cannons.
Which means you'll ultimately need two HGUC Primrose kits because of those winch cannons. (Ugh...)

Underside

The parts that mount the shield boosters on both sides of the backpack aren't included in this kit,
so you'll need to borrow them from the HGUC Advanced Hazel.

You'll need two enhanced shield boosters and two chest armor pieces.
Which ultimately means you need two HGUC Advanced Hazels and two Primroses.

Underside

Now let's tackle the ultimate AOZ Lego-fest — the Hazel Owsla Gigantic Arm Unit.

To do this, you'll need to remove the forehead, backpack, shield, winch cannon connector, and other parts from the Hazel Owsla.

In the end, the kits used to build the Gigantic Arm are as follows.
1. Hazel Kai (main body + backpack, head antenna)
2. Advanced Hazel (front skirt, feet)
3. Psycho Gundam (arms)
4. Hrududu II
5. Primrose ×2 (need two each of the winch cannons, chest armor, and shield joint parts)
6. Hrududu (Long Blade Rifle + cockpit unit)

Alongside the FG Primrose.
The sculpts are fairly similar, but there's a big difference in color separation.
Most notably, the FG version doesn't include the Hazel Kai backpack.

A comparison of the extra parts for building the Hazel Owsla.
If you look closely, the FG Primrose on the left only includes one set of shoulder parts,
so you'd need two kits to properly build the Owsla.
It got a lot of flak for that back in the day. Leaving out just one set of shoulders — seriously... -_-