Notice ὁ ὢν always functions substantivally, and often starts a new sentence in the GNT —
ὁ μὴ ὢν μετ’ ἐμοῦ κατ’ ἐμοῦ ἐστιν, καὶ ὁ μὴ συνάγων μετ’ ἐμοῦ σκορπίζει.
Matt. 12:30
Ὁ μὴ ὢν μετ’ ἐμοῦ κατ’ ἐμοῦ ἐστιν, καὶ ὁ μὴ συνάγων μετ’ ἐμοῦ σκορπίζει.
Luke 11:13
μονογενὴς Θεὸς ὁ ὢν εἰς τὸν κόλπον τοῦ Πατρὸς, ἐκεῖνος ἐξηγήσατο.
John 1:18
Ὁ ἄνωθεν ἐρχόμενος ἐπάνω πάντων ἐστίν· ὁ ὢν ἐκ τῆς γῆς ἐκ τῆς γῆς ἐστιν καὶ ἐκ τῆς γῆς λαλεῖ. ὁ ἐκ τοῦ οὐρανοῦ ἐρχόμενος ἐπάνω πάντων ἐστίν·
John 3:31
οὐχ ὅτι τὸν Πατέρα ἑώρακέν τις, εἰ μὴ ὁ ὢν παρὰ τοῦ Θεοῦ, οὗτος ἑώρακεν τὸν Πατέρα.
John 6:46
ὁ ὢν ἐκ τοῦ Θεοῦ τὰ ῥήματα τοῦ Θεοῦ ἀκούει· διὰ τοῦτο ὑμεῖς οὐκ ἀκούετε, ὅτι ἐκ τοῦ Θεοῦ οὐκ ἐστέ.
John 8:47
Ἐμαρτύρει οὖν ὁ ὄχλος ὁ ὢν μετ’ αὐτοῦ ὅτε τὸν Λάζαρον ἐφώνησεν ἐκ τοῦ μνημείου καὶ ἤγειρεν αὐτὸν ἐκ νεκρῶν.
John 12:17
πᾶς ὁ ὢν ἐκ τῆς ἀληθείας ἀκούει μου τῆς φωνῆς.
John 18:37
ὁ Θεὸς καὶ Πατὴρ τοῦ Κυρίου Ἰησοῦ οἶδεν, ὁ ὢν εὐλογητὸς εἰς τοὺς αἰῶνας, ὅτι οὐ ψεύδομαι.
2 Cor. 11:31
Ἰωάνης ταῖς ἑπτὰ ἐκκλησίαις ταῖς ἐν τῇ Ἀσίᾳ· χάρις ὑμῖν καὶ εἰρήνη ἀπὸ ὁ ὢν καὶ ὁ ἦν καὶ ὁ ἐρχόμενος, καὶ ἀπὸ τῶν ἑπτὰ Πνευμάτων ἃ ἐνώπιον τοῦ θρόνου αὐτοῦ,
Rev. 1:4
ὁ ὢν καὶ ὁ ἦν καὶ ὁ ἐρχόμενος,
Rev. 1:8
καὶ τὰ τέσσερα ζῷα, ἓν καθ’ ἓν αὐτῶν ἔχων ἀνὰ πτέρυγας ἕξ, κυκλόθεν καὶ ἔσωθεν γέμουσιν ὀφθαλμῶν· καὶ ἀνάπαυσιν οὐκ ἔχουσιν ἡμέρας καὶ νυκτὸς λέγοντες Ἅγιος ἅγιος ἅγιος Κύριος ὁ Θεός ὁ Παντοκράτωρ, ὁ ἦν καὶ ὁ ὢν καὶ ὁ ἐρχόμενος.
Rev. 4:8
ὁ ὢν καὶ ὁ ἦν, ὅτι εἴληφας τὴν δύναμίν σου τὴν μεγάλην καὶ ἐβασίλευσας·
Rev. 11:17
ὁ ὢν καὶ ὁ ἦν, ὁ Ὅσιος, ὅτι ταῦτα ἔκρινας,
Rev. 16:5
——
2 Cor. 11:31 really seals the deal. …Now I understand why these charlatans were coy about openly declaring that they believe ὁ ὢν functions adjectivally at Romans 9:5.