1 Corinthians 15:26-28 explicitly have the Father as "God", but that because he [the Father] put all things in subjection to him [Jesus], it shows that Jesus became equal with "God" his Father in relation to the "all things" [the whole creation].
Notice that all things will be subjected not to God the Father in order for God the Father to be "all in all". So Jesus appears to be essential to the eschatological fulfillment of God the Father becoming all in all [i.e. that all creation may be one with God the Father]. Again, all creation will be subject first to Jesus and that's the only time all creation will be subjected to God the Father. Thus, this text coheres with another Pauline text which speak of Christ being "all in all" (Ephesians 1:10, Colossians 3:11).
The manner by which he [God the Father] will subject all things to him [Jesus] also shows that Jesus is ontologically [by nature] God. Philippians 2:9-11 speak of everyone -- in the heaven, on earth, and under the earth -- will confess Jesus is Lord [κυριος - Adonai, or YHWH]. Jesus is the fulfillment of the monotheistic eschatological event in Isaiah 45:23. Only when everyone in all creation acknowledge to God the Father that Jesus is YHWH that God the Father is glorified. This coheres with the Johannine text which says that God the Father glorifies Jesus and Jesus glorifies God the Father.
Thus, in conclusion, 1 Corinthians 15:26-28 show that Jesus is both functionally God and ontologically God, and that these are his unity [oneness] with God the Father. Without Jesus Christ, God the Father would not have all creation reconciled with him and glorifying him. And Jesus wasn't a mere agent who does things to give glory to God the Father (a one sided relationship) since God the Father himself "super-exalted" Christ above/beyond "all things" not only in this text but in all the Pauline corpus. Both the Father and Jesus receive glory by glorifying each other (as in the case found in 1 For 15 wherein God the Father subjects all things to Jesus and Jesus gives the kingdom to God the Father and then subjects himself). The catholic orthodox church teaches that this refers to the perichoresis of the Trinity wherein the divine persons are in unison and united in everything as one God in the qualitative sense.