You might have got confused by reading the title of this lesson.
There is no such rule named “ઓ-ઈ-ઉં-આ-ઈ-આં” in Gujarati grammar. It is a rule of thumb. I have mentioned it here for you to remember it easily.

You will see that in Gujarati language, forms of adjectives, verbs etc. are created depending on gender and multiplicity.

And thumb rule is, if
masculine singular form ends with sound ઓ(o) then
Feminine singular form ends with ઈ(I)
Neuter singular form ends with ઉં(uM)
Masculine plural form ends with આ(A)
Feminine plural form ends with ઈ(I)
Neuter plural form ends with આં(Am)
So in short, ઓ-ઈ-ઉં-આ-ઈ-આં (o-I-uM-A-I-AM ) rule !!

e.g.

e.g. Black -> કાળો (kALo )

Black horse ->  કાળો ઘોડો  (kALo ghoDo )

Black chair  ->  કાળી ખુરશી  (kALI khurashI )

Black plank ->   કાળું પાટિયું  (kALuM pATiyuM )

Black horses -> કાળા ઘોડા  (kALA ghoDA )

Black chairs ->  કાળી ખુરશીઓ  (kALI khurashIo )

Black pages ->  કાળાં પાટિયાં  (kALAM pATiyAM )

Similarly if,
masculine singular form ends with sound યો(yo) then
Feminine singular form ends with ઈ(I)
Neuter singular form ends with યું(yuM)
Masculine plural form ends with યા(yA)
Feminine plural form ends with ઈ(I)
Neuter plural form ends with યાં(yAm)
So in short, યો-ઈ-યું-યા-ઈ-યાં ( yo-I-yuM-yA-I-yAM ) rule !!

e.g
I saw a boy -> મેં છોકરો જોયો (meM Chokaro joyo )
I saw a girl -> મેં છોકરી જોઈ (meM ChokarI joI )
I saw a book -> મેં પુસ્તક જોયું (meM pustak joyuM )
I saw boys -> મેં છોકરા જોયા (meM ChokarA joyA )
I saw girls -> મેં છોકરીઓ જોઈ (meM ChokarIo joI )
I sow books -> મેં પુસ્તકો જોયાં (meM pustako joyAM )

As you will visit further lessons, you will understand the usage clearly. As of now just keep this rule in mind and revisit it when you see its application.